Negative test, early period, tiny clots

Hi ladies. Recently had unprotected sex and took a plan b (around two weeks ago). I got off of the pill in October and cycles were normal since. Sunday around midnight I started my period (I believe) around 6 days early. I’m having really bad cramps, heavy bleeding with regular large clots and very small tissue/clot type “materials”. Also took pregnancy test that all day negative. Does anyone have experience with this being side effects of plan b? Should I contact my dr? Trying not to be panicked but a little freaked out. Thank you.