I had a positive ovulation test on tuesday, i have started my period today, five days after. how can this happen??

Not meant to be: Sometimes a developing egg just might not be a good one and the body knows this and the period will come sooner. More technically you may have had a poorly developed corpus luteum that was not sufficiently giving enough progesterone to support the uterine lining, leading to early bleeding.

Test not accurate: "Many women think that they ovulate on day 14, but 14 is an average, and most women will actually ovulate on a different day of the menstrual cycle. Your day of ovulation will vary from cycle to cycle.. Some women claim to feel a twinge of pain when they ovulate, but many feel no sensation at all and there’s no other sign that you are ovulating. See link for additional details. Feel good!