Mine randomly limped for a few seconds while at the park the other day, i think he just fell on it funny and it hurt so he limped a few steps and then he was back to normal...
Yours may have pulled something or done something so it hurts when he walks on it, I'm sure its nothing serious if he isn't crying about it and you can touch the area without problem...
See what the vet says but I wouldn't stress just yet!