A urine pregnancy test should usually be positive by this point three weeks out from your last missed period. Therefore, I think the chances that you are pregnant are relatively low. However, you should see your primary care doctor
about this issue, as a more sensitive blood test might be needed just to make sure.
Your doctor will now also want to think about other causes of delayed periods other than just pregnancy. In a young otherwise healthy woman your age, the most likely cause would be an anovulatory cycle, which occurs when the ovaries, for whatever reason, fail to release an egg at the appropriate time. If, on the other hand, you have a history of very irregular periods, then a condition called polycystic ovarian syndrome might be a possibility.
Your doctor can perform a complete physical examination to help determine whether or not any additional work up or treatment is needed at this time.
Also, if you are not pregnant, do not forget to use protection while having sex, because even though you have not had your period, this does not mean that you couldn't get pregnant!
Please set up an appointment with your doctor at your earliest convenience.