What Causes Delayed Periods?

Hi,I'm 17 and I had my period on feb27 and ended on march5th so I thought I was Gonna get my period on march but I didn't get it,Yes I'm sexually active.and April came I went to get a pregnancy test it came out to b negative.now it's the end of the month and I still haven't gotten I mean I still cramp and evrything here and there. I don't have any pregnancy symptoms and I have that gooey stuff comeing out of my vagina. I don't know if I should be worried? Please help me