pregnancy symptom?

I'm worried I could be pregnant even though I have had a few negative pregnancy tests and a couple of periods. I only just finished my period this weekend and for the last 3 days my nipples have felt funny, like a bit Pingy and tingly. Why could this be happening. I'm on the pill and my boyfriend wears condoms, I havnt had sex for a few months and have periods (on time) since then. 
Please help