HELP! bleeding after sex- not a virgin

The night before last I had sex with a guy I've been casually seeing, I hadn't had sex for a year until then. 
It wasn't painful, it took a minute to get in but it didn't hurt. 
Then last night at the end of the day I was getting in the shower and noticed a small amount of dark blood in my underwear, I'm over a week away from my period and I'm on NUVARING so my periods are always regular. Today when I woke up there was blood again and it was brownish pink. 
I know, super gross tmi but I'm so confused why I'm bleeding?? ANYONE help!! Please!!

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