I had sex for the 1st time. did not bleed. 2 days heavy clear vaginal secretions. now brown to black odorless thickening discharge. probable causes?

Unsure: That you did not bleed is quite normal. That this discharge is odorless is probably good but I'm not able to imagine why you'd have such a dark thick substance coming from your vagina. No pain? (good) If it was non-fresh blood, it would not be odorless. I'd see a doctor for an exam but I expect it may not be related at all to losing your virginity. Don't worry - just check.

Most likely... : Most likely cause is vaginal infection so get evaluated with a GYN provider for exam and testing. It is also wise to test for pregnancy if sexual encounter was more than a couple of weeks ago.