Newly "fixed" boys fighting and spraying

Hi all
Long time reader and first time poster.

On to my problem.I have 3 cats all of them are approx 10 months old.@ boys and 1 girl.The boys are my problem right now.
They were neutered 8 days ago.Before the neutering they were very sweet ot each other and as far as I can smell had not sprayed yet.However about 4 days ago(4 days preop)one of them started spraying just about everything in sight the other one consistently since then does the taildance into an air conditionening vent.
Also 2 days ago those 2 started fighting like cat and dog so to speak.
What gives in the past fixing them always seemed to lessen/prevent this kind of behavior.
Just to give you an idea they fight so hard they actually draw blood on each other,altho not huge amounts or serious,and they seem to be claw injuries not bites.Boht of them walk around almost constantly with big bottle brush tails.They are still very nice to people (altho one of them seems to be mad at me for taking him to hte "bad" place as he still remembered a week before he had blood drawn lol I am sure with enough treats he will get over it )


Another problem is that altho my cats are fairly big they are also vey very skinny (but according to the vet extremly healthy muscular and fit with no bodyfat).
Well one of the boys and to some extent the girl came back well almost emaciated looking s an example he went in weighing 9 pounds 4 oz and came out weighing 6 pounds 15 oz.
Now I understand being without food or water for almost 24 hrs accounts for some wieght loss but geesh he looked terrible.I have never seen this extreme.

Right now he is looking somewhat better his fur got its sleek shine back he eats well and his gums look good as does his nose (cool and moist).He eats well altho no more than before to make up for the weight loss.
So what can I do to encourage a little weight gain?They are free feeders and alays have food and water readily available.

Thanks
Kat

The effects of neutering are not immediate after surgery. In my case, Wawa continued spraying for a month and Ashley (outdoor stray) stopped after a few days. I do not know why.
My problem though is the fighting. Wawa still quarrels with Ashley. It's been going on since May. Because I had a similar experience with Joji and Skinny (they fought each other for months) I am hoping Wawa and Ashley will settle their differences in time.

Simply put, they are behaving as though they have never met and are two males fighting over territory. Since they weren't neutered until so late, they have learned all the not so pleasant male bad habits of spraying and fighting. Since they were seperated and taken to a vet and then brought back, they behave as stranger cats because they now smell completely different. Male cats don't lose the hormones ntil about a month after being neutered so unless they rememebr each other and accept each other as one time buddies, you have about a monh to deal with this. Rememeber don't punish the cats. They are doing nothing wrong, only reacting with natural instincts. Buy some Feliway sprays and Plug ins, put them all over the place and put Pure Vanilla Extract on both of the cats and see if that helps. It should.