Why would my period suddenly get shorter and alot lighter?

My periods have ALWAYS been very very heavy and lasted from 5 to 7 days and for the last 4 months they have been spotting and only last 2 to 3 days. I don't take bc (me and my hubby are trying to get pg) I'm not under any stress and my diet hasn't changed, I have taken hpt after hpt and all neg. My doc said i wasn't ovulating and i got those opk and i did ovulate this month so I just wanted to know what could it be?