I'm worried because I had my period earlier this month (ended on the 13th) and started another period on the sat 28th morning, it only lasted for a day or three then stopped but occasional starts again - during sex and when I went to the gym. I'm not stressed (about anything other than this:p ) at the minute and I haven't been on the pill since before xmas... could I be pregnant?

It is possible you could be pregnant and that sex is irritating the tender cervix. It would serve you well to make an appoinment with your physician for a physical exam to be sure that everything is okay.