Decrease in milk supply 2 months

Is it normal to see a decrease in milk supply after 2 months?
I've been BF baby on demand or every two hours. I then would pump to make sure my breasts are "empty". Lately I've only been able to pump 1 oz total from both breasts. Usually I'm able to get 6 oz in total. I eat well & drink lots of water. I'll be returning to work in two weeks. 
What could be the reason a dip in my supply?
Is it the stress of returning to work the reason?