I think that a cheap pregnancy test is worth the peace of mind! This is totally anecdotal, but I do know more than one person who has become pregnant on birth control. Birth control is very effective, but no form guarantees 100% safety. With certain types of birth control, the risk of an ectopic pregnancy is higher, so that's a good reason to be aware of any change in symptoms. I would take a test, and in your case, I'd probably buy a digital with a simple yes/no answer so that you can move forward definitively. Either way, I hope you feel better!