Reasons my period is late?

I missed my period last month and i need some answers as of why. My period has always been regular, ive had one since i was 9 years old. Ive researched some reasons like stress/ extreme athletic activity/ eating disorders/ medical conditions/ etc but they dont apply to me. The only change ive made in the last month was dislocating my knee and having to take celebrex for a short amount of time. One of the serious side effects of the medicine is loss of red blood cells so i thought there was was a chance thats linked to my missed period. I am sexually active but i dont believe im pregnant. Anyways, any help?