Oh poor Cocoa. Did you check her pads, there's no tears or anything? One of the dogs I walked was like this, fine when running around but limped when walking, it's weird. The vet said he had split pads and we had to do salt water washes and put vaseline on them 3 times a day. If it's a muscular thing you can do hot/cold treatment, I wrote up about it here:
Keepers is hurt!
Don't know if any of this helps at all. Rest is probably the best cure, but that's $64,000 question - how do you get a boxer to rest????