If i had infectious mononucleosis and it went away, is there a chance that i actually have herpes instead?

No: Herpes and mono are unrelated clinically. Having one is independent if the other.

Likely just mono: I am guessing that you've developed sores or ulcers in your mouth or other sensitive areas. Mono can cause extremely painful ulcers in the mouth or genital area. While it's possible to be infected with both epstein-barr virus (the typical cause of mono) and herpes at the same time, it is more likely that the sores or ulcers are due to the mono, assuming that mono was correctly diagnosed.