If she hasn't eaten in 3 weeks, then she doesn't have anything to push the poop through her system.
A snake can go months without pooping. My snakes don't poop unless they have consistent and regular meals. If they go off-feed, they stop pooping (for obvious reasons). My snakes are fed once or twice a week and some of them don't poop every week. Maybe once a month.
Is the yellow discharge poop or urates. Urates are yellow/white hard stubstances. That is normal for snakes. It can be watery when it first comes out sometimes.
So unless your snake isn't acting normal or looking ill, I would think your snake is ok.