At 3.5 months, babies get to be much more efficient nursers and just don't need as much time to nurse. Both mine started out needing like half an hour or ore to feed but by the 3-month mark, maybe 3.5, they were sucking down all they needed in 10-15 minutes, and it got to be less and less as time went on.
Green poops can also be caused by an intolerance to something in the mother's diet. Dairy and soy are common culprits, but there can be others. My second had an intolerance to carageenan, of all things, which turned out to be in the rice milk I was drinking since I can't tolerate cows' milk. She didn't really have any of the other symptoms of intolerance like gas/wind or tummy pain, but the spinach-green poop was unmistakeable and went away almost immediately (or lessened a lot) the day after I switched brands of rice milk to one without carageenan. Had it been dairy, it likely would have taken longer to get out of the system.
If your DD has enough wet and dirty diapers, I'd say she's getting enough to eat overall. If she's just gone thru her 3-month growth spurt, then your breasts will likely adjust the foremilk and hindmilk balance on their own, meaning the green poop may be caused by something else.