I am on birth control, and do occasionally take a pill late or forget it completely till the next day.. my period is always normal (28 day cycle) until this month. On May 10th I had a headache went out to eat with friends and after taking 3 bites of my sandwich I felt nauseous on the way home and puked 2 times on my way home and again when I got there. I've been feeling nauseous, experiencing slight breast tenderness, and cramping. My period was supposed to start May 24th started 3 days late but it's very light bleeding I no longer feel nauseous and breast tenderness is gone.
Could I be pregnant, or have been.. and am going through a miscarriage..

It could be anything you suggested or another medical problem.

Birth control is not 100% effective. "Forget about it" or skip dosages and you run the risk of pregnancy.

Only time will tell -