Why is my period so light?

I've been on the pill for some time now and at the very end of my period this month I had sex. 10 days after the sex I had some light spotting and I was worried that I might be pregnant. So I stressed about it for a week and I started getting stomach pains. They weren't pains from my vagina, it would just be stomach pains. When I stopped stressing they would go away unless I ate food that really upset my stomach. Anyway last night I got my period and I was very surprised because they usually don't come until about 24 hours later. I could definitely tell it was my period, but it's so light that it isn't staining the pad I'm wearing. Whenever I wipe when I go to the bathroom I see the blood so I know I'm having my period. It's never quite this light though. Also this morning I woke up and was feeling very sick. My back hurt and my head and stomach hurt and I was getting very light-headed when I would stand for a while I also had a fever and lots of diarrhea. I'm feeling much better now but I still am having some diarrhea. I couldn't be pregnant right? Is my period light because I was stressed and then got sick? Or do you think I could actually be pregnant? Thanks for the help!