Pongy Cat!

Oscar has been on a chicken and rice diet for about a week now, to help his colitis, and since he went on it, he has become quite pongy! He seems to fart everytime we're near him and it's not pleasant!
He also hasn't pooped in a couple of days (pee's regularily though) but I haven't seen him strain at all so I don't *think* he's constipated.
Does anyone know what might cause this and whether it's a cause for concern>

Thank you in advance.

Well, it is not ever a GOOD thing.

I have never heard it called that. Might have my daughter try that term since she was told yesterday by her teacher she should not say fart .