Could my mom be developing dementia?

Consider, with this recent change, a test of the urine for UTI. These can cause mental changes. If not, and if this continues, you are on for checking this out with your mom's MD. Good luck.

My Mom was sensitive to penicillin. It can cause the following.

Penicillin and other antibiotics can cause neurological issues, including delirium, in some cases. Delirium symptoms can include confusion, hallucinations, seizures, and muscle twitching, and can appear within days of starting treatment. These symptoms usually go away within days of stopping treatment. 

Call her doctor.

Seventy-six is pretty young to have dementia, unless it's Alzheimers. But if there hasn't been any other odd or inexplicable behavior, then I agree she needs to be accompanied to an appointment with her primary doctor. You can slip the doc a note discretely telling who you are in relationship to her, and her worrisome symptoms. While there request the HIPAA Medical Representative form so that your Mom writes in you or your sister's name and she signs it and turns it back in to the receptionist. This allows her medical team to legally share her private medical information with you without your Mom being present.

There are many other medical conditions that can seem like dementia but are not. This is why she needs to be seen and have a thorough exam.

Mom needs medical evaluation as soon as possible, even if you need to make up a fake reason to take her. Don’t contradict her or argue about what she believes, it only frustrates you both, but do get her seen. Explain to the doctor away from her what’s been happening

This sudden a change in behavior may very well be a UTI, dehydration , stroke , pneumonia , a reaction to a new med or a ton of other things . Many other problems can cause confusion , delusions , hallucinations in elderly besides dementia . I would take Mom to the ER to be checked out. Tell them what has been happening the past two days .