std??? fiancé possibly cheating

I pray I'm over reacting and this is nothing but Iv been with my fiancé about 4 years now, Strickly him only !! Not to mention we've been ttc for about a year. 
Well this morning I woke up got in the shower on my morning routine and felt a pain under my left butt cheek close to my vagina .. 
I got in the Mirror and saw an open sore with little bubbles around it in a perfect circle. I wanna post a picture but don't need back lash Help!!