Mrs Sharon vickery

I have a four year old jack Russell that has not been done he has got a fantastic temperament and has never cause any problems before, I also have four cats that he has recently started tormenting, all of our cats have been done apart from one female that is having kittens now, but he's mostly tormenting the adult male, that has been done. Any ideas of what's happened, and why he is suddenly acting like this?