late period, starting to freak out.

So I wasn't really freaking out but nowww i am!!! My period is late 18 DAYS! I have cramps , headaches, tender breasts, period like symptoms. I had PROTECTED sex, i took a pregnancy test to be sure and it came back negative. Im also taking new medications and every week I go into the doctors office to get shots. Could the medicine be making it come late or what's going on??? Im freaking out now..