I started back at the gym when DD was 4mths old. I used to feed her beforehand so I was empty (and so that I could put my sports bra on ).
I am not sure why you were gushing out, other than perhaps you had a let down while you were exercising, I used to get let downs when I thought of DD.
You willl find your supply will be fine, just keep going as you were and she will get plenty.