It could be anything - that is the danger of the internet - you get too much information.
It could be simply your stress and your worry about the possibility of being pregnant that you feel nauseous and getting stomach pains.
To ease your mind - you can go to the store and get a pregnancy test but it may be too early to get an accurate read yet - it is suggested that you wait a week after your "missed" period or one - two weeks after sex for a more accurate reading.
As far as the "period" you are having - it does not 100% rule out pregnancy, technically you would not have a period if you are pregnant, but you can have bleeding so that is why it can be tough to tell and some woman do not think they are pregnant.
I think the suggestion to call planned parenthood or at least a doctor - not sure what Brook is - assuming a medical center? You could call there and I would imagine they would direct you where you can help. Maybe just calling planned parenthood or a doctor or nurse would help put your mind at ease.
Caveat - we are not doctors on here so we cannot provide medical advice - just our experiences. Googling things are just going to stress you out more.
My best advice is to talk to someone with medical knowledge - my other advice is not to stress - personally I think your chances are small you are pregnant between taking the pill (even if not at the exact hour) and that you are showing signs of having a period - it does not rule out 100% - as abstinence is the only thing that is 100%, but your chances are low.