Spotting when you're supposed to be on your period.

Sarah
I was supposed to start my period on March 9th, it has been two days and my period has been extremely light, and today, (March 11th) I barely have any blood. I'm not sure if I should be worried or not.
I am on birth control, have been for 9 months. I am pretty consistent on taking it at the same time every day but sometimes I do take it a few minutes to hours late. 
I am sexually active, but have never had unprotected sex. I always use a condom.
I got a UTI the day before I was supposed to get my period. I know sometimes if there is bleeding during a UTI it can be pretty bad. I'm not sure if it was blood from my UTI or from my period. 
I'm not sure if I should be worried, or not. Nor do I know what it could be from. 
Please help me!!!!