This description you give is a bit unusual, as it sounds like you've developed a blister in this area where you previously had a mole. I would suggest that you go see a doctor
, as you will need to have someone look at the area to make a diagnosis. You don't necessarily need to see a dermatologist
for this. If you have a primary care doctor
at your school, or even an urgent care center, they should be able to fit you in and help you figure out what this might be.
Ruling out a sexually transmitted infection is of course important, although the location you describe would be a bit unusual. However, if you have been having anal intercourse, then a sore in this area might be more expected. Herpes is the sexually transmitted infection which is most likely to produce a blistered sore, although syphilis can also produce sores or ulcers. After examining the area, your doctor may want to send some basic blood or urine tests to rule out these possibilities.
If this turns out not to be a sexually transmitted infection, then it could simply be a blister (from irritation of the skin, for example) or some inflammatory skin condition. Your doctor, if they think this is what is going on. will give you additional advice about how to manage this. Good luck.