Unusual Period- Possibly Pregnancy?

I have recently been feeling a little different.I am 17 and I had unprotected sex a week ago on July 31(this was the only time in about a year) with my boyfriend and my period is due a week after that. He used the pull out method before he ejaculated(which I know is not very effective). I'm usually irregular and my period came August 8th, but this does not seem like my usual periods. Before and during my periods my breasts are usually tender or sore but not this time. Also I usually have a pretty heavy flow the first couple of days, but this time I do not. It's heavy enough for a pad but not the usual. I also noticed something else unusual. When I urinate there is no blood in the toilet. There is only blood on the pad and when I wipe. I was thinking maybe implantation bleeding, but the blood color is normal as if I was having my period. The symptoms I've been having are cramping, bloating, gas, and some constipation. All of the symptoms are usual during my period except the constipation. My last period before I had sex started on July 5 and ended July 10. Since the day I had unprotected sex I have been really worried about whether I could be pregnant or not. And I did not start feeling different until the the 5th of August. Could my stress and really worrying about being pregnant have caused these symptoms and lighter period. Or could I possibly be pregnant?