First overnight stay from memory care.

I think the simplest answer is - dementia is a progressive disease. As you have recently seen, the increased memory loss and confusion meant changes to her living arrangements and level of care. So may you have to adjust in your own routines with your mother. What worked yesterday may not work today and what works today may not necessarily work tomorrow. This recent change may be hard for you to accept as it means you lossing another bit of your mother - dementia is a crappy disease.

Yes, the tantrum was because you took her overnight. You can expect more tantrums if you do this again. Better to keep her focused on where she is. Been there. You do what is best for them, not what they throw a fit over.

My mom has been in memory care for 5 months and has become more agitated in the past couple of weeks - many new residents have moved in recently along with new staff
She has had several UTIs and a bad fall while there, and over the July 4th holiday weekend I learned through my personal caregiver that a facility staff member has been taunting her to spike her agitation - I reported him and gratefully the facility took swift action
I wouldn't dare bring my mom home for an overnight stay as that would be cruel but up until the last couple of weeks took her out for dinner and an occasional movie each Sunday - her behavior outside is acceptable
Despite her age (nearly 93) and limited mobility she too loves to go out primarily to eat - she hates where she is but has no short term memory and has had many bad falls - tried caregivers at home but it just didn't work
I can barely stand being at the facilty for a few hours at a time so I imagine it's terribly hard on her
Even before her dementia she would try to escape rehab following a fall or surgery so I don't dare move her to assisted living
Very frustrating and depressing for everyone especially those poor souls who ate much younger and could end up spending years in a facility

People with dementia can have a bad day with no discernible reason. (There may be a "reason" in their convoluted way of thinking, but we may never discover it.)

I'm not clear ... have you taken your mother for an overnight since she has been in memory care?

A lot of changes...
Taking her for an over night is probably not a great idea. She is getting used to being in her room. She knows what it is like and she knows her routine. Taking her for an over night she has to get used to a new place a new routine and new people around her.
The fact that there are new people moving into the unit where she is just confuses her even more. It has to be frightening all of a sudden seeing new faces and hearing new voices. Imagine your memory not being real great and all of a sudden you have a whole new group of people living with you....do you know these people?...were they here before? and you just forgot..how could you forget that many people? I would think this would be rather frightening.
If possible can you arrange to stay the night with her for your over nights rather than bringing her to your home? This might give you a great perspective on how the meals are, what the activity level is day and night. (Many facilities allow residents limited access 24/7 so if someone is up and wants a bowl of cereal or ice cream they can get it. This is their home and if they are not on a restricted diet there seems to be no problem) Some facilities do have a "Guest Suite" that is available for a minimal fee if you can not stay in her room.

I think it's hard to figure out why someone in Memory Care acts a certain way. I now know that they may react differently at any given moment.

I learned that my cousin gets disoriented and confused when she leaves her unit, so I don't do that anymore. Just a short time out of her unit and she was confused when I returned her and didn't know why I had taken her there. (She's been living there for 2 years.)

You might try shorter visits and not overnight to see if that helps.

Can I ask - what type of behavior prompted the move from Assited Living into Memory Care?

Short term memory not very good and confusion is why she moved to memory care.

My mom is in a memory unit, although her long term memory is excellent. The structure is key for her. She lives by the clock and wants to know what is going to happen when. Usually memory units have more structure and a smaller population....so this works best for her. If she gets out of her routine to go the doctor or something else we have "high" anxiety.

So many good thoughts!
I found out yesterday from her main care giver at the facility that mom woke up the day after I took her back not feeling good. She slept part of the day and had cough drops. The caregiver there said it was not the overnight stay because she did not exhibit behaviors caused by the stay with me. She just felt achey and needed rest.
Sometimes I think there are more changes in the unit then when she comes to my house where she knows everything and relaxes. Like a mini vacation. I am very fortunate the facility is new and her main caregiver is wonderful! She knows they all have hard days and how to work through each situation to calm them.
When I talked to her caregiver yesterday mom was back to enjoying her friends feeling physically better