Has anyone experienced this?

I started taking the pill back in January. My period has always been on time and regular. In july I started having alot of nausea and headaches. I also became very sensitive to odors. My period was late (1-2 weeks) and I decided to take a pregnancy test. I actually did 5 on different days and at different times and they were all negative. One of these was done at a clinic. Now I am a month late! Should I be worried? Is this a side effect of maybe taking the pill for so long(7-8months)?