It still sounds like an STD to me. You could have gotten it from oral sex or from being masterbated by your partner. You should go to the doctor and have it checked out. It could be nothing, it could be serious. It's best to know and get it taken care of as soon as possible.
I can't tell you what causes it, my friend has them and is becaus eshe has STD's. you should seriously get check, you could go to: Planned parenthood or any other clinic.
Good luck.
Please do not be embarresed to see a doctor they see this all the time this is why they are there. Alot of people experiece this and it is not an std. Please read the question WHAT CAUSES VAGINAL SORES
The blisters may also be caused by a reaction to an antibiotic drug you may be taking such as Cipro family of antibiotics. I had one of the rare reactions to this drug. It is miserable.
I have had blisters checked out by doctors who have reported back that it is NOT herpes. My cause is vaginal dryness, and intercourse literally "rubs you raw" just like you would get a blister from wearing a shoe that was too tight.
You can have std but maybe you dont i heard the blister on you vaginal hert p.s i forgot how to spell hert
Here's what I do know. If you've been sexually active and not used protection there IS a possibilty it could be HSV 1 or HSV 2 (two forms of the herpes virus). BUT you really have to go to a dr to find out what's going on. IF you do have herpes, you need to get treatment. If not, then you still should be treated for whatever it is. It's a very delicate area so if it's from friction, I suggest using a good water based lubricant when you're having sex and during foreplay.