Could possibly be a CCL injury (especially because of the part where you notice the limp after resting); weirdly common with dogs in general but especially with pit bull type dogs.
Just FYI allot of vets will try to push surgery right away (and if it is a serious CCL injury it is usually necessary); however you can heal a minor CCl injury with conservative management - which as others have said involves strict crate rest along with anti-inflammatory medication, massage/range of motion exercises; joint supplements (and then slowly building the muscle back up through controlled exercise).
If it turns out you are dealing w/ a CCL injury there are a couple of really good yahoo groups, one on conservative managment and one on surgical options.
My girl had an injury last year from agility and we were able to heal her completely with CM; only thing is I did retire her from agility
no point in taking chances and risking a surgery. But basically she does everything she wants to do; I just don't push her to do allot of jumping, quick running turns etc.
Ames - Mel is the cutest thing eva!!