He may be having bouts of gas. Sometimes the motion of a car ride can help with that, which might explain why he was suddenly better after coming back from the vet. Do you keep infant simethicone on hand? That can really be helpful with gas. And there is no danger or any side effects from it, so even if your bunny didn’t have gas, a dose wouldn’t harm him. I’ve noticed that some of the brands have artificial sweeteners, which I think are dangerous. I use Mylicon, which does not have artificial sweeteners.

Instead of giving him the pellets all at once, you might want to break it up and give him a spoonful a few times a day.  Some people scatter the pellets inside the hay if their bunny eats pellets too fast. Also, see if this is happening after giving him certain greens or fruit.  There may be some that his digestive system can’t handle.

I agree that something is upsetting his stomach. Can you describe his diet in detail? How old is he? Has he been tested for coccidea? 

When you suspect gas, have you felt or listened to his tummy?

Medications can also upset his tummy. You should put together a bunny tummy first aid kit: simethicone (infant gas drops), critical care for if he isn’t eating or needs fluids, thermometer, benebac (a probiotic, which can help after antibiotics). One of the dangers of gas is temperature fluctuations, so taking his temp during an episode is important (often they need to be kept warm).  

What type of veggies are you feeding? You might want to restrict his diet to hay and water, and maybe a tiny amount of pellets, and carefully add each one in and see if he has a negative reaction. 

Hmm, collards and bok choy are in the cabbage family, so have been known to cause gas in some bunnies. Other’s report their bunnies react poorly to lettuce. 

You might try only feeding dandelion and cilantro until he gets stable again, and then try adding in little bits of everything. I don’t feed my buns anything that isn’t leafy, except as a very small treat, so you might want to cut the bell pepper too. Carrot tops (stems and leaves) are also good for upset bunny tummies, if you can get them. Fennel is also good for gas.  Try to get him to eat as much hay as possible, and encourage water drinking by providing a bowl and serving veggies dripping wet. GI stasis and gas are kind of a chicken and egg thing, one can lead to the other. 

If he has another relapse, and you can rule out a dietary cause, ask the vet to test for coccidea (just need a fecal sample). It is a parasite that can cause diarrhea and other gi issues in young bunnies, and most rabbits have it or have been exposed to it. 

How old is he? 

Mint leaves can also be good to soothe tummies, if your rabbit will eat it. I grow spearmint. You can usually find the plants in the produce section of a grocery store, and they can be grown in pots outside.

You may need to do some detective work with the greens and veggies he is getting, to see if one or more of them might be the cause of his stomach problems. Just pick one or two of the normal veggies you give and only give that for 3 or 4 days. If those don’t seem to bother him, try two others for a few days. He might not like the fact that he isn’t getting the variety he normally gets, but it would be good if you can determine if one or more of the veggies might be the culprit.