could I be pregnant?

I had what seemed to be just a very very light period about a week ago but now I'm starting to wonder if I need to be worrying that I might be pregnant? The past week or so I've been feeling nauseous, feel like I can't eat most the time, and light headed but I've also been having mood swings . I also feel like I'm kinda having cramps and been abnormally constipated. My friends think it's the heat but the more I think about it or look things up I'm wondering if it could be something else. Anyone dealt with something similar?