I think something is wrong with me.

Okay so I know the chances are slim and I am sorry for bringing this up because I know none of you are doctors and can give me a for sure answer! I am just looking for some advice or possible similar experiences. I am on the last day of my period and it has been my second period since the last time I had unprotected sex with my fiancé since he left for work. I took a test before my last period and got a negative. I still have this weird feeling that I could be pregnant and have back aches and nausea for the past month. I have heard of some women having their full on periods during pregnancy and I am starting to think that could be me. Should I spend the money to take a test or just save my money and realize if I was pregnant I wouldn't have gotten my period. I sound desperate I know :(