I have a small blister in my vagina, what cause this?

Herpes, genital warts, human papilloma virus, vaginal trauma, poor nutrition/ stress (usually causes a single painful blister kind of like what you're describing), clothing, bacterial infections, cuts from shaving...

Actually, lots of things could cause it. If you've been sexually active I'd lean more towards the STD cause, if not it could be something environmental.