Prenatal vitamins lengthening Cycle.

I have always kept track of my cycle even prior to TTC. For the past two years, it’s always 26 days.

I’ve been TTC for two months now and this last month I have been taking prenatal vitamins for the first time. Could they make my cycle longer?

I am having very mild cramping from time to time. My breasts feel heavy and sore. I took a hpt and it’s BFN.

Should I retest if AF is still late in a couple of days? Is it really possible to be pregnant after a negative hpt when AF is two days late?