What Causes Delay In Menstruation Other Than Pregnancy?

I am 6 days late, and have taken 3 pregnancy test so far (two of the same brand and one of a different) and all have came up negative..I am on birth control Tri-Spintec. i have been under a lot of stress recently with work, but my home life is just fine. could i still possibly be pregnant? or is it stress related?