Its been 5 days past my period due date, I had taken a test and it came up negative. Could I still be pregnant?

You could be pregnant.
Are there any other reasons you think you are pregnant? When was the last time you had sex? Use a condom, spermicide, birth control, etc.

It could also be an irregular period.
Are your periods always regular? How old are, and how long have you been having your period? Did you recently get on/off of birth control? Any other hormone altering drugs?

There are too many unanswered questions to give you a definite answer.

As the last resort, no one here is an over-the-internet-doctor, and we will just recommend going to see yours anyway.

Had sex about 2 weeks before period. Off birth control around 3 months ago but regular period since then. 18 yrs old had period since about 12

You could and the hormone levels aren't high enough, or your period could just be late.

After fertilization and implantation, the unterine lining is maintained by hCG (human chorionic growth hormone) produced by the embryo which basically is just supposed to keep you pregnant by making sure your ovaries keep pumping out progesterone. Most pregnancy test kits check for this hormone because this is the only time it would be present in urine.

Initial levels are pretty low until further development of the embryo, so wait a few week or two and test again and just keep an eye on your cycle.

It's possible you could be pregnant.It depends when you last had your period.

Hormone levels may not be high enough. I would go get a blood test!