Why does my French Bulldog sometimes collapse with momentary paralysis?

Sometimes for no apparent reason my dog will just collapse and not be able to move for a few moments. He just goes limp, and sometimes he'll urinate as if he can't control himself, but I'll pick him up and put him back on his legs, he'll be a little shaky at first, but then pretty much back to normal walking around as if nothing has happened. If I left him alone, I'm sure he'd probably be able to move around again on his own, but it's scary for both of us, so i try to get him back on his feet as quickly as possible. He's a little over four years old, not overweight at all, and he's never had any major health problems. I've read French Bulldogs could sometimes have problems with their backs, and it's only started to happen in the last week, at least three times. He used to get sore if he was having a hyper day and did a lot of jumping on and off the sofa over and over. The next day he would move really carefully just like a person would the day after doing some activity that's more strenuous than they're normally used to. But he's never collapsed with momentary paralysis. It almost seems like it happens right after he's made a sudden movement after not moving for awhile? Goes totally limp, then everything is fine. He's got me worried. Does this sound familiar to anyone?