Sorry to hear you are having some issues "down there". May is ask what you mean by saying you were a "bit hypo down there"? I am not sure what you mean by that.
I did a little research to see if I could learn something to help you, and I found some interesting information. First of all, it does not have to be an STD necessarily. It could simply be a vaginal blister, caused by friction, in grown hairs or clogged sweat glands.
For this type of blister a hot bath or a sitz bath is the best treatment.
Now it the blisters are in a cluster, (more than one) that could be sign of herpes. If they itch or burn, you need to see a doctor.
If your blister has a hard edge, it could be a syphilis chancre. If it is painful and has a hard edge, it also needs to be checked by a doctor.
Other blisters can be caused by HPV or genital warts.
So you see, your blister could be caused by any number of things, but is not always a sign of a STD.
Getting a proper diagnosis is important, because some of these conditions do need medical treatment right away. I would encourage you to watch it carefully for a day or two, and then depending on what you see, you can decide to get seen by a doctor. Being so young, I would always be safe and let a doctor see it for the proper treatment.
I am a bit concerned when you mention that you and your boyfriend have had unprotected sex. I really hope that is your last time. it only takes once to be exposed to a sTD that you will have the rest of your life. YOu only have one body, so please treat it with great respect, and be very careful who you choose to have sex with. Everytime you have sex with someone unprotected, you are having sex with everyone they have had sex with too.
I wish you the best