DAY CONCIEVED???

So December I was supposed to get my period the 4-9 ... ended up getting a heavy flow once on the 1st then little to no spotting till the 3rd. This month according to that “period” I should of got it the 1-4, so far still no sign of it. I am going to the dr on Thursday so waiting till then to test but has anyone experienced this? & if so would you believe my date of conceiving to be from December or January!