I was on birth control for a couple years, stopped taking it and had unprotected sex a month later. I took 6 birth control pills after in replace of a plan b. I bled for a day about a week later but now I feel nauseous in the mornings sometimes and lower back pain and bad headaches. Could I be pregnant?

If you had sex without contraception then yes you "could" be pregnant, do a pregnancy test or see your doctor to confirm it though, ignoring it could leave you a mother.

Taking 6 birth control pills at once is not a good idea... so why would you stop taking birth control pills, then have unprotected sex, and then panic about getting pregnant? Sounds to me like you wanted to get pregnant.

So to answer you... yes, you could be.