A patch of my cat's fur fell out?

My cat got 2 surgeries about 2 weeks ago. one was neutering, and the other was umbilical hernia. he got both surgeries in one day.

after about a week and a half, i could tell that he was showing more of his aggressive side. well, he seems to be showing his soft side more as well. at one point he'd be all on his back and showing his belly, being all lovable, and then later on he'd be (kind of playfully) biting and get really hyper. he doesn't hiss or look angry, just in a playful way he bites and stuff.

oh and he's 1 year and 4 months old.

It just looks like the fur's been pulled off. it's just a red color that seems to be the skin's color- it doesn't look like an irritation or anything.

He's the only cat in the house and he's an indoor cat. never goes outside.

Oh, and also we don't know what happened, but sometime in the week, his left front paw thumb's nail broke off. when the scab of the blood was gone, we saw the skin part of the nail, and it seemed to be healing. it doesn't hurt him much now, so that was good. But i think that could have stressed him a little? I can't be sure..

The patch of the fur that came out is located on his right front shoulder, like right beneath the neck. it's about an inch in diameter. He's a domestic short hair, white and black tabby.

He doesn't seem to have any other patches lost, but we hope to not get more patches missing..

I've gone around the house to see if i can see where the patch fell, and i've seen a few patches here and there around the rooms. one downstairs, one upstairs..

What could it be? is it stress? is it something serious? We usually don't have time to take him to the vet these days, so we try to find out the reason first. so please understand.

I'm really worried. Right now, he seems bright, just trying to play with the toys he has.

He eats fine, the same kibbles he's been eating very well. they're adult kibbles of course.

Oh i don't know if this could be one, but we feed him some snacks and one of his favorites is the Nutri-Vet Hairball Formula. (I'm just trying to give as much information as possible here)

He sleeps a LOT. as much as he plays a lot, running around with me and all, he sleeps A LOT. which i don't think is a big deal, since i know that cats can sleep up to 21 hours a day..

he's not obese nor skinny. He's normal. The last time we went to the vet to get him neutered, they said his weight was average.

That's all i can think of right now. but if you would like to know more, i will put the additional info down. please let me know what the most possible reason could be.

The most likely and the least likely would be great.

Thank you SO MUCH for your answers. all answers will be appreciated!