should I be worried?

should I be worried about being pregnant if I've had 6 negative pregnancy tests but still experiencing sore nipples / boobs? it's been about 4-5 weeks since the last time I've had "unprotected" sex. I am on the pill & pretty sure I never missed one or was late while I was sexually active. could my breast tenderness be because of the pill itself? I'm so worried. I don't want to spend more money on pregnancy tests :(