What Causes Delay In Periods With Negative Pregnancy Test Result?

Hi My name is Jessica Twigg. I'm 18yrs and weigh 89ish kilos and i'm 5" 8. I'm wondering whats going on with my menstraual cycle. I had my first child just under 2 years ago. About two months ago i came off the pill to start trying for a another baby. My first period after coming off the pill started 16/6/2011 and went for three days the second started 2/7/2011 and again went for three days. According to my calculations it should have started again on the 19/7/2011 but alas it did not. Now im 10 days late. I've taken 3 pregnancy tests the last one being on the 27/7/2011 but it was negative. I'm not sure what to do next or whether i was taking the test to early because my cycle only seems to be 17 days?