I have been having changes in my period. I used to be on a 27-29 day cycle and I could predict exactly when my period was coming. But for the last couple of months it has been more like a 21-24 day cycle w/ my PMS symptoms coming a little over a week instead of 3 days before my period starts. The only thing I have changed during this time is I stopped taking my lithium. Could that cause this change? Is this something I need to worry about? I've never had this short of a cycle before, even before I started taking lithium. I don't know if I should worry.