Roof of mouth looks like a huge bruise, it's almost all purplish. i'm on antibiotics for infected tooth. what is this?

Call the doctor: There are a couple of things going on here. First the antibiotic you are taking may not be effective against the infection. Are you taking the antibiotic as prescribed? You need to call the perscribing doctor about this and he may change the antibiotic. Bruising on the roof of your mouth can be caused by trauma. Have you had anything in your mouth that could do this?

Injury?: Did you injure yourself? It could be a bruise. They can happen in the mouth. It could be "something else". You should go see a dentist for a limited exam. Your insurance should cover that.

Needs to be seen!: There are lots of possibilities and we should refrain from trying to guess what you have over the internet. I highly recommend that you call your treating dentist who is aware of both your medical and dental history and ask if you should be seen. Your own dentist may well have the answer for you. It might be as simple as some bruising from the injection or perhaps related to the infection.