What could cause my period to be late?

I have frequent periods even though I have PCOS. I actually have short cycle ranging from 14-18 days long. I started taking Vitex to try to lengthen my cycle and also to try to get me to ovulate since i never do. Well after 3 month it definitely extended my cycle 5 or 6 days longer. However I had to stop taking it because I was still not ovulating so my doctor wants to do clomid with IUI. Well I am now CD 29. I have never gone this long before without a period. I don't think I ovulated this last cycle like always and I was actually wanting to see my period for once so I could start the round of clomid. But instead nothing. I have had some very very light spotting every now and then when I wipe but then goes away since around CD 18. I tried a pregnancy test yesterday on CD 28 and it was negative. So what could be causing this all of a sudden? I am not stressed or anything. Could it be stopping the Vitex that is messing my period up?