Cycles and Suppliments Please someone HELP ME!!

right im needing help.
usually my cycle is 24 days long. but for the past month ive been taking a pregnacare suppliment designed especially for conception which includes the suppliments l-arginine, inositol, n-acetyl cysteine, betacarotene, selenium and the usual mix of vitamins and minerals. i read up on the previous ingredients and i cant remember which one but one of them is ment to help fertility by extending your luteal phase. this months cycle was 20 days (AF started today going by past months it shouldve started on this thursday coming) i dont know why this is! any ideas anyone??
i was thinking is it the suppliments working by shortening this months cycle to extend next months cycle so that the luteal phase is longer??? the reason behind thinking this is cos my cycles usually 24 days long but this month its 20 so that the 4 days misssing off this month will be extended onto next month to give me the average 28 day cycle?? or am i just wishfully thinking?
arrgh ive no idea why this has happened and its starting to bug me. and also its upset me a bit cos its told me 4 days earlier than usual that this month is not my month.

please if anyone knows anything about the above mentioned suppliments can they tell me??

Thank you in advance

Rachel xxx