In the last month there have been times that my mom can't support herself, stand and walk. She has fallen to the floor as I catch her.

Have the doctor check her for Orthostatic hypotension. It is a sudden drop in blood pressure upon changing position. My stepmom suffered from it and it caused her to fall and break her hip.

You need to do orthostatic blood pressures. This sounds like blood pressure drops with positional change. It is quite common. Often seniors have to take medications that change blood pressure, lowering it. When they stand up their pressure drops. The secret is slow positional change and standing before "zooming around". That's one thing to check out. I would take blood pressure on this senior three times a day if able.
That's my guess. The rest of it is in the hands of the MDs. If this continues, as for PT consult in the home to work with Mom with this problem in mind.

Drug interactions can cause strange symptoms and can "easily" be discounted as the cause. For example, I recently read somewhere on this forum that if one is taking BP meds, they should NOT be taking pain relievers like Naproxen (Aleve) because it "undoes" the effects of the BP meds. So, maybe make sure that drug interaction is not what is causing her issues. You can call her pharmacist to discuss.

Also, I see you made a different post at the same time, outlining a different health issue. Why didn't you combine both of these health issues into one post? I feel like we're only getting bits and pieces of info and this makes giving helpful advice more difficult...and possibly inaccurate.

https://www.agingcare.com/questions/why-is-my-mother-shaking-so-much-466785.htm?orderby=recent

#1 thing I want to mention here DO NOT CATCH HER.
You can help her gently to the floor then call emergency services for a "lift assist" if there is no transport to the hospital 99.99% of the time there is no charge. (some areas do charge) The problems with "catching" her is that you can get hurt yourself. Please watch videos on how to help someone to the floor.

This could be a blood pressure problem. I take medication for high BP and there have been a few times where I have become light headed and have gone down. I usually "feel" it happening and have gone down gently or I will sit and let it pass if I happen to be near a chair. Or Just holding on to a counter works.
It passes very fast so by the time someone would take my BP there would be no indication that there was a problem. If she is on BP meds and has lost weight or become dehydrated that might be the cause.
If you happen to have a BP cuff at home use it as soon as you see this happening but as I said it can pass very quickly, once down on the ground where the head is level with the heart or close to to level the BP will normalize.

Foxx
Does your mom have dementia? My DH aunt, 94 does. She can be needing assistance to get up from her chair, to walk with her Walker, to get out of bed. Especially when her aide is in sight. Then I will see at other times that she will sit right up, get out of bed, change her pull ups with no assistance. Overall the helpless times are winning out, she is declining, but she still has her moments.
I have also seen her in the past not be able to walk across the room when her therapist asked her to. Later that day she would easily be able to walk the same distance when she was focused on what she wanted rather than what she was asked to do. It was like thinking about walking was too complicated for her.
In other words, it’s not physical. It’s mental.
Take your mom to a geriatric primary or a neurologist. Physical therapy can be ordered by her doctor and that’s also helpful.

foxxmolder, check all of your Mom's prescription meds and see if any have a side-effect of being dizzy or lightheaded. The doctor can prescribe a different brand of the medicine to see if that helps.

Have your Mom's primary doctor give Mom a check-up [I know you were at the hospital, but the primary doctor would know your Mom the best]. Maybe it is a case where Mom just gets too tired mid-day to stand-up. Or it could be positioning blood pressure changes when going from sitting to standing. Or it could be low blood sugar at that time of the day which can make someone feel faint.

Hope the doctor solves this mystery.