Cheated On in the Past. Lip Sores. Could This be an STI?

My So has admitted to cheating on me last year several different times with at least 3 other women. I was unaware when it happened and only know because he confessed to about one. A few weeks ago he was drinking again and admitted that it happened with 2 other women besides the one I knew about and that he had paid them both to do anal because I was uncomfortable with it.

About a week ago a small painful wound formed on the inside of my bottom lip. It was really painful and I went to the doctor thinking it was just a canker sore. They did not do anything to examine it except look at it and I was told it is a mouth ulcer caused by something viral. They gave me amoxicillin and antiviral meds but a few days later another sore appeared on my upper lip on the inside. I was told to keep taking the meds and it should clear up over Christmas.

My grandmother is concerned that I should see a different doctor and see if maybe this could have been caused by something sexual. As far as I know, the last time he cheated on me was a year ago but I have no way of knowing if he still is or not because he has a drinking problem, has cheated on me in the past, and he is still pressuring me to do sexual things I am uncomfortable with. Any advice would be greatly appreciated.