Milk Supply Imbalance

My LO is 3 MO and started daycare about a week ago, she has been exclusively breastfed up until now. I noticed a while ago that my left breast produced less when I pump bit never thought anything of it because my baby was content. But now that I am back to work Ive noticed that I cant barely even get one ounce out when I pump! My other breast pumps 3-4 oz each time. Whats most strange to me is that my left breast is leaking and looks ready to be pumped, but then barely anything comes out! Then after Ive tried to pump for 15 minutes, if I remove the pump and put pressure on my breast, the milk just pours out! Its like I have milk in there, but I cant get it out. HELP!?