Ovulation problems

Before i started on pregnancy vitamins, my periods were perfect, 28 days, never late. Now that i am on pregmavit and started to try concieve, my periods are all over the place! Last month i was 5 days late, then aunt flow arrived. Now it has been 17 days since the start of menses, and still no positive on the ovulation tests, and cervix is still hardish... But lots of CM!! What gives? I don't feel that stressed?? Could it be stress?