Had unprotected sex and irregular periods. now i am bleeding but have light cramps and i usually have painful cramps. why is this?

Irregular: Since your periods are irregular then one can not predict when they are going to come or what the cramping is going t be like from one cycle to the next. Consider starting birth control to help improve your regularity and give you control over your cycle as well as control over when you get pregnant. Ibuprofen is often helpful for cramping