Pumping output at 7 months

So my little guy has been ebf and is 7 months almost 8. I have been back to work since he was about 5 months. I work part time from 7-1, 3-4 days a week. I feed him right before I leave for work and I pump once while I am working for 20 min.   I had been up until recently been able to pump between 6-8 oz!.  Within the last week I have been getting between 4.5-5 oz.  not sure why this has happened. I have been trying to eat healthy but have been still eating when I am hungry and drinking plenty of water.   Any ideas lady's?!? Is this just a normal time to drop off a little, or was I overproducing before?!?  Or is there something I should do to increase my supply?   Thanks any advice would be appreciated.