Blister is herpes. Not that you definitely have herpes, it could be like you said from friction or a pimple, or some ther reaspn. But, the other STD’s would not cause that problem most likely, I think the doctor just did the routine testing for STD’s because you had become sexually active again. there are some rare STD’s that I am sure you were not tested for, that can cause ulcers this like that, but I don’t think you have to worry about those. They are very rare found in third world countries things like that.
If you get another “blister” I think you should go immediately for a herpes culture. The culture is very innacurate after days have past.
Ok, but let’s assume it isn’t any sort of STD. The suggestion I have is to rinse the area well when you urinate. Not because I think you need to rinse the urine, but to keep the area free of vaginal discharge. A bottle with a straight stream of water that rinses the area is easiest to use. Take a bath in the evening and morning, can be just a quick dunk of your vulva in shallow water for a couple of minutes. You can also try using tampons to keep discharge off the area. Just do it for 4–7 days and see if there is improvement. If there is, in my opinion it is not a dryness problem, but something with the discharge.
Ointments will help protect and hydrate the area, but also can clog pores and cause pimples possibly. However, sometimes ointments do help my vulva heal, and protect from discharge. Also, areas that have been inflammed, using OTC cortizone cream or oitnment has been very helpful.
I am not a doctor, just a girl with shitty luck since my 20’s battling this crap. Again, I have no real idea how similar your problem is to mine, but the water rinsing has no downside, I think it can provide a lot of information, and at least for me I know wihtin days its working.