I would definitely suggest that you go to see your primary care doctor
or your OB GYN doctor about this issue. Some of the symptoms that you are having, including the heartburn, fatigue, breast soreness, and even spotting, could be signs of pregnancy.
The pregnancy tests that you get in the store and perform on urine are quite good tests, and most of the time when they are negative this means you are not pregnant. However, early in a pregnancy, these tests are not quite so good. Since it has only been two weeks since the last time you had unprotected sex, it is possible that your are pregnant but that the urine test is not turning positive. In this situation, a blood pregnancy test, which your doctor can order for you, should settle the issue.
If you turn out not to be pregnant, then your doctor will also be able to help you figure out what else might be going. For example, depending on where you are in your monthly cycle, you could have some fatigue and vague symptoms with spotting at the time of ovulation. Many of these symptoms could also be caused by a common viral infection, like the common cold.
Talk to your doctor!