What Causes Severe Pain While Urinating During My Periods?

Hi.. Please help me.. Every time am on my period I seem to get some sort of an infection, am fine the other 3 weeks of the month but While on my period, for about 2 days straight I get terrible pain while peeing, it is like peeing razor blades..I also have a feeling that I need to go all the time so I spend most of the 2 days in the bathroom. Am very confused as this only happens while on my period. What could it be? Thanks