Are you up to date with all STD/STI checks? If not, then I would get checked.
I didn't use protection that day, but later on I did and the same thing happened. Im on birth control so my periods are always on track. It's really odd.
It could have just been caused by irritation from sex, especially if it was too rough or you weren't lubricated enough. Could have also been due to you ovulating since you're around mid cycle. Did you use protection?