Blood after sex..?

I just ended my period 3 days ago, my Fiancé and I have had sex during my period and every day after with no protection. 
The day after my period ended we had sex and I had a sharp pain in my right lower stomach area, with deep penetration... It's not the first Time we've had this issue, but now every time we have sex, I get a little brown/reddish color in my panties AFTER we have sex. Nothing during. I also just over a UTI... If that matters. 
What could this be and why? 
I'm a little nervous about it.