My cat now cries often when he uses the litter box

I'm not sure what's wrong with him, but I know it's related to the litter box. If anyone has any ideas, I would really really appreciate the help.

General Info: My cat is a neutered male, 7 years old, and overweight. He's been eating the same food for four months now (one of the all natural brands) and has been using crystal cat litter for the same amount of time, both of which he and his sister have been completely fine with. I know he's eating because he has a special feeding time, and I've noticed him drinking a couple of times but haven't really been watching who's drinking from the water bowls; I just keep refilling them every day. I keep the litter box really clean.

His symptoms started two weeks ago. He does not do this all the time, but sometimes he whines or meows or even runs out like he's scared when he uses the litter box. I'm not always sure if he even just used it; I just hear him crying and go over and see him walking away from or next to the litter box. He's just in the general vicinity. Two of the times, I checked right afterward and he had defecated the first time and peed the second time. Neither time had blood (I think). He's also been picking on his sister, which he never does, so there's irritability and such involved
I brought him to the emergency vet one night a week ago, and they told me that they did not find anything wrong with him just through a physical and he seems fine. They said that there was no urine in his bladder, so there wasn't a blockage or something, and they kept feeling his stomach and pushing it for a while - I don't know why - but they didn't find anything.

Hi mountaingoats,

Welcome to the forum

If he's been checked by the vets and nothing amiss was found I wouldn't worry too much about this, tho' I know that's easier said than done. Even tho' he's used to using it, it could be something as simple as him taking a dislike to that type of litter, so trying another sort might help, as may giving him his own litter tray.

Just as with food, they can suddenly go off using a certain litter for no good reason that we can see, so trying a different sort is the easiest thing to try first.

Ian

Hi Mountaingoats, pleased you've joined us.

As Ian says, they can take a dislike to litter even though they've been using it with no trouble before. I would alsoo give him his own litter try, don't know if you're aware but it's usully good for them if they have their own trays plus an extra one somewhere els in the house.
Two cats together can occasionally take a dislike of the other, or one of them will be bullying the other one. Now we may not think they are but cats are devious, they can often sit in front of the tray so that the other cat can't get past them to use the tray. This all ends up with a stressed cat. Could be something like this is going on, worth a try anyway. Hopefully, these small changes will help.