Light menstual period , negative test

Hi. I was wondering if I could get some helpful advice. In December, I had unprotected sex with my boyfriend. Although I know it's not fool proof, I looked at multiple ovulation calenders on-line and it turned out that we had sex (on different days)on my most fertile days. I was schd to start my period on January 5. I've typically always have normal periods so when I didn't start on the 5th, I took a pregnancy test at a local pregnancy clinic on the 6th that came back negative. Of course I started my period on the 7th. I normally have VERY bad cramps with my cycle but this time I didn't; just back pain. My period only lasted about 3 days. When I took the original pregancy test I would have only been about 2 weeks pregnant and according to different baby websites, there may not have been enough hormones for my test to come back positive if I was pregnant. I'm still wondering if I pregnant because I'm still have back pains and I have been having nausea every night for a week (I understand that some women have "night sickness" instead of morning sickness). I'm always tired and my appetite has incresed. I'm normally a light eater but lately I seem to always be hungry. I also seem to be more emotional than normal and I have been having headaches. Oh, and I have been going to the bathroom a lot. Does anyone think I should take another pregnancy test? I would be about 3 weeks now...I'm only concerned because I would hate to think that I wasn't pregnant when I really was because I would want to start prenatal care. What do you think?