Strange periods

Hi ladies, my husband and I have been TTC for the last 8months. I've been tracking my periods and ever since I came off birth control 2yrs ago you can set you watch by them. The last 2 months at first I came on as normal but was really light and then 2 weeks later the same happened and so on. I have taken pregnancy tests but they are negative and I have no other symptoms. Anyone any ideas what could be happening?