cat - weight gain and lump on lower back

Question
About 3 months ago I switched my cats diet to Indoor formula Cat Chow, it says on the label it will help to maintain a healthy weight for indoor cats, but my 8 year old male neutered cat has become very overweight.  I was also petting him yesterday and I noticed a soft moveable lump, it almost feel liquid filled it is about the size of a marble and it is on the lower back right where it meets the tail, sort of near the base of the spine, but on his back, when I touch it - it makes him twitch, scratch himself, bite himself, and even nibble at my hand.  It almost seems that whatever this lump is that it is putting pressure on his nervous system, I have other indoor cats and I thought it might be a bite from one of them, they do play rough sometimes.  This same cat has had a chronic cough all his life, sometimes he coughs as much as 4 times a day, sometimes it ends with him vomitting, but only spit comes up.  Different vets have put him on steroids that seem to help for a few days or even weeks and then the cough comes back.

Answer
Robin, I tried the indoor formula and my cats didn't like it, so I did not feed it long enough to make a weight difference, but if your boy is gaining weight, I would try a reduced calorie diet.  At 8 years, his metabolism is slowing down and he probably doesn't play as much as he used to.  I do not know the fat content of the indoor formula.

I would have the lump aspirated to see if it is a seroma (clear fluid) or an abscess (puss).  It could have happened when they were wrestling.  I don't think it is anything to worry about.

Have you ever had x-rays to see if there is anything growing in his throat to cause irritation?  Just an idea.