My mom doesn't rememver going to the bathroom when she had gone just a few hours earlier. Is this a common dementia behavior?

Get calming meds for mom from her doctor. OCD behaviors go hand in hand with dementia, and bathroom issues are high up there on the list. You cannot reason with mom anymore, even if you show her a photo, so don't bother. She'll accuse you of lying next, so there's no point. Pick up a copy of the book Understanding the Dementia Experience by Jennifer Ghent-Fuller on Amazon so you can learn about dementia and how to best understand your mom's mind now. The 36 Hour Day is a great reference type book so you can look up specific behaviors she's exhibiting and see how to respond. Stick with the forum, too, we can often be more helpful than the doctors bc our collective experience with our demented loved ones is priceless.

Good luck to you.

Obsessive behaviors is very common with dementia, and can be treated with medications, so I would talk to her doctor about getting her on some that will help.
Your moms brain is now permanently broken and the best thing you can do to help her is to educate yourself more about this horrific disease of dementia so you're better prepared for what is ahead, as things will only get worse.