Period was 5 days now 3

Hi, I was looking for some advice please....

My period are normally 5 days, bleed for 3 days , then on the 4th day nothing sometimes spotting then bleed again the 5th day....they are ALWAYS the same every month. The past couple of months my period has only lasted 3 days, and is lighter. We only properly started to try for a baby 2 months ago, only things that’s different is I’ve been taking prenatal vitamins and I have been trying to track ovulation. Has anyone got any advice? I don’t feel stressed about ttc, but I think is it a coincidence that my periods have changed since ttc? Would prenatal vitamins affect periods? Am I subconsciously stressed about it? I’ve done pregnancy tests too and all negative. Has anyone experienced the same thing/got advice please?

Thanks in advance xx