Drying up already?

Manal • Soon to be a mommy of 3 under 3
My daughter will turn 3 months old on the 12th of April & I exclusively Breast pump after I failed at breads feeding her.
I been doing great and keeping in the freezer but in the last 4 days I noticed that I don't have enough milk & had to supplement from my freezer stash. 
I usually would get a bad day when I can't put in the freezer, but never like this.
Is it possible that I'm drying up already???