On birth control and should start period today but the blood wont come out of my vagina there is blood it just wont come out?

Mechanicalvshormonal. You may have heavy lining if your estrogen/progesterone balance is altered which results in heavy bleeding, and sometime blood clots that can block your flow. Fibroids in your uterus can also block the blood. You should see your ob/gyn or primary doc because there are other more serious causes like endometriosis or adenomyosis.
Hypomenorrhea. Sounds like a very light flow, which is not unusual for a woman on ocp. If you have been taking your pills every day, then i suggest simply tracking your cycle on a calendar and seeing what happens next period.