hard stomach

Hi I tried to look online, but did not find anything <I guess I am not very good at researching> but I have the following question?
What are the possible reasons why a cat would have a hard stomach?
Our 3 year old Jakub <Happy birthday cause he just turned 3> is veery healthy, but yesterday evening I noticed that his stomach is a bit harder than usual <this happens to him when he goes to the vet, at least that is what every vet told us, they say it happens because he is stressed>. It is not swollen or anything, he is not in pain <believe me, I would know, he is a very vocal cat>, he goes to the toilet as usual, he plays, he eats and drinks, he does not have a fever, and he is well hydrated. We recently <maybe 10 days> switched his food from Nutro to Merrick, since I thought that the quality is mych better and the recall is just frustrating me <btw, no he did not eat any of the contaminated brands> .. Any ideas?
Thanks

Based on everything you said, my first thought is gas, my second thought is worms.

Is it more "round" (distended) than usual?

Hmm, gas, I did not think of that but it does make sense. The Merrick food contains a lot of fibre rich ingredients <lot of vegetables and fruits, grains> which is new to him a bit. And he does enjoy when I massage his stomach gently, normally he hates it when somebody touches his stomach.
The worms, I do not know, he was tested some time ago, and he does not come into contact with anything that would infect him in my opinion..
Thanks
Oh and it appears normal, it is not more rounded...