9 days late, negative pregnancy test, whats going on?

So following my last question, I took your advice and did a pregnancy test. It was negative. I'm so confused, my periods normally run like clockwork and I have all the symptoms, tired, moody, queasy, lightheaded, heartburn, constipation, more frequent urination, low back pain.. The thing is, I'm 15, just coming up to my GCSE's, and I really don't want to be pregnant, any idea's what's going on?