What Causes Irregularities In Menstruation?

I have trying to get pregnant since last September and I been having regular periods and having intercourse on the days that I ovulate and still I get a negative test . And this is the month of July and I m having a period but didn t have one in June . What could be going wrong ? Could it be when I had an std from a long time ago relationship and didn t no for like two months but I got rid of it !! Please help me !?