Hi Abby and family! That's terrific that things have been going so well. It's always good to hear about a Tripawd making such a good recovery.

I'm not a vet but I suspect that she's been getting too much activity. What does her day look like right now? How much walking and play time is she getting? At three weeks out, it's sill very early in recovery and you should still be making sure she takes things slow. I know that's super hard with some dogs, but it's important to build up stamina little by little. When a dog loses a rear leg, they lose half the propulsion to move forward so her remaining leg is doing twice as much work. She is probably very, very tired at this point. 

Try massaging her hips and lower back, and inner thigh, to see if she's sore anywhere. You can also try a heat and ice therapy treatment on the area where she seems sore.

While it's not unusual for new Tripawds to do too much too soon, it's also something that we need to be careful to avoid. Learning to manage her activity is a process, and soon you will be able to know when she's about to get too much of it. But for now I would recommend scaling way, way back on whatever it is that makes her so tired.