Severe abdominal pain during period but also pregnancy symptoms! Suggestions?

I am on day 3 of my period but yesterday I was having severe abdominal pain and vomiting and since then I've had soreness/pressure on my lower abdomen. The last time I suffered this type of pain I found out that I was pregnant the next day. I don't want to go out and buy an expensive pregnancy test (the cheapest one in any drugstore in my town is $11) just to discover it was nothing. Is there any physical possibility that I could be pregnant and still have my period? Or that it could just be severe implantation bleeding at the same time I was supposed to get my period? Any help with understanding this would be greatly appreciated!