What Causes Blisters On Vagina That Are Sore And Tender?

Hello - Iv recently found blisters on my vagina that are sore and tender. A couple have broke releasing a blood like liquid. They appeared in the Space of a day after wearing tight jeans. However me and my partner have been experiencing quite rough and vigorous sexual encounters which left me quite sore. Iv had no prior sti s - and neither has my partner - however he had a couple of blisters on his hand which we believed to had been dermatitis. Help?