Dad passed away and now Mom's dementia seems to have advanced dramatically. Any advice?

Bereavement counseling can be helpful. Medications are good when a person has a chemical problem causing depression, but what you describe is a situational depression and quite normal for someone who has lost a spouse.
She has spent the better part of her life as a wife. Even in a less than perfect marriage, there was security in that. There was a reason she stayed all those years...in spite of the difficulties she needed him, now he is gone. I imagine she feels confused, lost and fearful. Grief is something that each person moves through at their own pace. The best you can do for her is to listen with an understanding heart.
I'm sorry for your loss.

My mother with dementia became worse in the time surrounding my father's death. They had been married for over 64 years. Not having him around was a real shock to her system. A few other things happened. After about a week following the funeral, all the family and friends stopped coming around or calling. She pulled more into herself and began over-medicating until she let me take charge of her medications. Over-medicating was probably her way of dealing with the loss.

Do you have siblings? You may want to encourage them to stay involved while your mother works through these changes. What is your mother doing during these energy spikes? I wondered if she was trying to busy her hands and mind.

My father has been gone 3 years now. Mom still misses him, but I think that it is better. Sometimes now she says she can't even remember what he looked like. We have pictures around, but his memory is fading to her. Mostly now she talks about either the present or her childhood. She used to call my father "Daddy," but now when she says the name, she means her own father. It is odd how things evolve after such a huge change in the life of someone with dementia. Tired, I would just be there for your mother and step in with medications only if you see there is something pathological in her grieving his loss.

I have to agree that 2 weeks isn't much time for your mom to have adjusted to your dad's death.

If her behavior has changed she might need to see her Dr.

Two weeks is waaay too soon to expect "adjustment" -- even for someone who doesn't have dementia.

Let her talk about your dad if she is inclined to. Acknowledge her feelings, whatever they are. That might include sadness and relief and guilt and regret and fear. Just listen.

This might be a good time to look through old photo albums or scrapbooks with her.

Reassure her often that you will never abandon her and you will see that she is always safe and well cared for.

Is her energy spike causing problems? Discuss the symptoms/behaviors that concern you with her doctor. The intense feelings during mourning do not necessarily need to be medicated, but if she is a danger to herself or others some intervention may be needed.

Any drastic (and sometimes not-so-drastic) change can trigger an increase of dementia symptoms.