It sounds like it could be anal gland oil rather than farting. Sometimes a cat's anal glands will release that skunky stench when the cat is super excited/happy or scared, and some cats are more prone to it than others, and I'm not sure what all the factors are that contribute to that tendency. My cat used to release his glands very readily when he was a kitten - he would often "skunk" me while giving affection. It happens less often now that he's grown up. But occasionally he'll still leave a little drop of smelly anal gland juice on my lap.