What Causes Dark Red Blood In Menstrual Flow?

I'm right on time for my period, but its not normal.. Day one it appears to be dark red and clumpy. Just this past week I've been battling a pretty severe UTI and I was trying to remedy it with things from home, plenty of water, cranberry juice, etc. I wasn't sure if maybe the UTI had anything to do with this dark red blood that I'm assuming is my period.