No period since september 2009,

Hi, i have been having sex with my bf for a year and a half, we have always used condoms. However my period is 4 months late, i havent had one since sept 22nd 2009!!

I feel nauseated have a constant headache tender breasts, lower backache and have terrible mood swings, could i be pregnant. i did a test before chrismas, around the 17th and it was negative.

Any advice

Thanks