If you have been having unprotected sex, then there is a possibility of pregnancy.
The pregnancy tests being negative means you are probably not pregnant. Cheap tests work just the same as pricier tests. The symptoms you are describing could be PMS.
If you don't start your period in a few days, take another test. If it is negative, you are not pregnant. Or, you could go see your doctor and get a blood test.
Planned Parenthood gives FREE and accurate pregnancy tests.