Jennifer,
This is exactly what was and still is happening to us. A few days before period, Anna wanted to nurse more often and sometimes acted as if there was not much milk there. At first, I thought, she was teething and that's why she was so fussy but, later, I figured out that this was always before AF.
I am taking Fenugreek tea when she starts acting like that and I know my period will be soon. I usually take one cup of it (in a month) and things seem to be going back to normal. I didn't want to start Fenugreek capsules, they are more potent than tea because my supply wasn't too bad. Another thing that helps is letting dc nurse longer or more often if he is cooperative enough to do that.
