milk supply

So my lo is 8 months old Sunday & I have exclusively breastfed him since birth. My supply was always great. Until now. I started my first period when he was 5 months & I have missed a period this month. Every time around the monthly my supply drops. It did the same this time except I didn't start but now it won't come back up. I'm drinking so much water, eating oatmeal, taking fenugreek twice daily. I'm starting to freak out. Has this happened to anyone? Yes I've taken two pregnancy tests too & both were negative.