First time pumping

I have been breastfeeding my LO this whole time whi is 5w3days and I decided to start pumping for when I go back to work & when I pumped 2 things happened... one- I hardly got anything on one side and the other it hurt so bad I couldn’t do it... what could be the problems? I know LO is getting enough when he feeds bc he has over 8 wet diapers and countless poopy diapers a day and he gained over 2lbs his first month appointment so I’m just a little discouraged bc if I can’t pump I don’t know what will happen when I go back to work when he’s 3 mo.