I am on birth control, though recently i have had tender breasts and my period was brown and fairly light but lasted 4 days. could i be pregnant?

Probably not, but...: It can be hard to tell if a woman is pregnant because some of the symptoms seen in early pregnancy can also happen in non-pregnant women. A urine pregnancy test gives a reliable result if done correctly. One can test about 3 weeks after sex (3 weeks after when conception might have occurred). An Ob-Gyn doctor can evaluate any persistent or worrisome symptoms.