18 dpo no AF yet but some cramps?

ive never made it past about 12 dpo, yes my luteal phase is very short which is why its been almost 2 years & still havent conceived. last month i started prenatal vitamins and hoped that would lengthen my luteal phase but maybe it worked too well because im on 18 dpo now and no AF still. i ovulated on cycle day 16 and my cycles are usually about 31 days to 35 days long. im not testing til im a week late for AF but can the cramps mean its well on its way? its kinda off and on which is normal for my PMS symtoms but its been like this for about 3 days. which that isnt quite normal. anyone in the same boat?