The initial weakness in your leg was because of the stroke, but you were improving with PT, so I don't think anything new is wrong. You have been experiencing general fatigue from the stroke, but your leg can get easily fatigued, too. I think you are just overworking it.

I think that it's important to talk to your PT about your fatigue. PTs are usually very understanding about these things and can make adjustments.

I think you can continue to improve with PT, but at a pace that is right for you. Pushing it too far can be a bad thing and usually doesn't help.