TTC totally changing my cycle....

My husband and I have been TTC for 6 months now. Before we started trying, my period was always on time and very predictable. Since we started trying, though, my period is sometimes a few days late, or a totally different flow from one month to the next. Last month it was 8 days late, then just spotting for four days, and now another week and a half later I started what seems more like a normal flow. Has anyone else experienced anything like this since TTC???  I've done dozens of pregnancy tests and they're all negative, so I don't know why all the major changes.