If you had lamictal rash you probably would also have it on your lips and face. They told me that if it is in the mouth it also is typically going to be in the throat and breathing issues will be there. Do the sores in your mouth itch? I had the rash and had it around my mouth and on my lips and was told emphatically that any itching in my mouth meant ER NOW until the rash was gone. But I don't think it usually starts in the mouth.

On the other hand you do not want to mess with it. When i had it I noticed a small rash on my cheek on afternoon and didn't think anything of it because I was out of the danger zone. So I took my meds that night without a thought and woke the next morning with a big rash that spread rapidly, ultimately covering most of my face, one eyelid so that eye was swollen shut for days, lips, and neck. But it might not have been so bad if I hadn't ignored the first signs.

So maybe hold the meds to be completely cautious and call an on-call dr tomorrow or wait for Monday? If it gets worse or you have any signs it is spreading and causing breathing difficulty or your face is breaking out then it is more emergent and maybe find an urgent care to get checked out. It's a hard call; most rashes are completely fine and only a tiny percent are THE rash. THE rash is distinctive but not every dr wil recognize it, at least that's my understanding, and that means it gets over-diagnosed sometimes. But it's best to not take risks no matter what, you don't want spreading.