The symptoms of pregnancy are very similar to that of PMS- when do you think you may have gotten pregnant?  Just because the test came up negative does not mean it is for sure- if you tested too early, the p.t. would not have been able to detect the HCG hormone that shows up when you are pregnant.  I would suggest waiting at least 2 weeks after the time you had sex that you think you got pregnant at, and then once more 2 weeks after that- assuming your period didn’t come.  To save money, I would further recommend going to a local crisis pregnancy center where you can get the tests done for free and they have a ton of resources to help you should you find yourself pregnant.  Good luck!