HELP!!!! When ever my dog bend over, he starts yelping?

My dog just out of nowhere yelped, and we narrowed it down to when he bends over, he starts yelping. I have no idea why, and it scares me a little since we recently lost our older dog. We think it could be a cramp, or maybe he got something stuck in his throat, but we are not sure. All we know is that in the last 5 minutes, something happened that really hurt when he bends over. Please help!