Cherry popped?

Tonight I had more of a deeper penetrative sex with my SO (we never really go deep deep) and we both heard a physical pop , we laughed to ourselves and thought wtf was that , ten continued , after the sex as he was disposing the condom he held it up to reveal to both of us that there was a bit of blood on the condom! Was this my "cherry" ? Or my other theory is that I am on my week of taking no pills (I am on a 21 day pill) and the sex has triggered a bit of the period to start ? Any opinions??