Feeling funny after period....

I have a question. My husband and I have been using condoms only as birth control, as we weren't sure we wanted kids yet but I don't react well to the pill. We decided last month we'd quit using condoms and see what happened. Well I started feeling weird, nauseous alot, dizzy, tired, and very moody, which is very unusual for me, I'm normally not that bad. I thought maybe I was pregnant, but then I had my period, it came at 27 days and I am normally 28 or 29, though it's not unheard of for me to be early...it was very light though and only lasted 5 days and I normally go the full 7. I figured I just got lucky this month lol bc I looked up implantation bleeding and I don't think it was light enough for that...there was some definite bright red blood the first couple of days just not as much as usual. But now I am still having issues with feeling nauseous and dizzy, and I have felt crampy at times, and I never feel that way right after a period. Took a cheap preg test for the heck of it and it turned up negative. I am wanting to know is there anyway I COULD be pregnant even though I had a period?