Period while pregnant?

I'm on the pill and last had in December.. I've had my period three times since then but it's been extremely light and I've slowly noticed my stomach getting a bit more fuller and rounder. The thought struck the other day, so I took an HPT and it was negative. I'm not sure if I'm just worrying myself silly or if I should be nervous about this -- I'd be abt 14 weeks along (geez!). Should I get a blood test? Is it even worth it or am I being foolish?