Getting discouraged already!

I went back to work on Saturday. I pumped 12 ounces total during my 8 hour shift. I thought that was pretty good, only to discover he had drank 14 ounces while I was gone. I shared paced bottle feeding with his dad. The next day, during my 8 hour shift, I pumped 13 ounces... Thinking that the paced bottle feeding probably helped a little, I was happy... Only to discover he had drank 16 ounces while I was at work! I'm getting a little discouraged that I'm not pumping as much as he's drinking and wondering how this is going to affect my supply in the long run, or his frustration on my days off. Does anyone else have this problem? Does anyone have any suggestions?