Zack acting really weird

Okay both our dogs ARE FIXED.

Now I have it really cool in here (its a hot day)

But for some reason Zack has been panting quite a bit and he keeps trying to mount Sasha.

She is getting frustrated with him and I'm worried about him. The panting mostly. (he is also refuses to use the bathroom outside...he just sniffs....I hope he isn't constipated again)

Anyways I can't keep him off her (and she doesn't fight him off she just tries to walk away and she will nip too. but he is really bugging her) I keep having to pull him off her

I have put him in a seperate area?

I don't understand this behavior (I mean he is 2 and he is a fixed and so is she)

Would he react this way if a dog in the apartment complex was in heat? (even though he is fixed)

He eats, drinks, and pees fine......

but he hasn't had a BM today and he just keeps wanting to hump (he never does this, occaisionally there will be the dominance humping but not like this)

Recently at vet and clean bill of health

Yes, my guess would be that Zack smells a female in heat nearby.

Some one nearby has an unaltered ACD, and when she went into heat, my boys were sniffing/drooling around the area where she pees frantically... and I could barely walk them away, because they were humping each other. LOL! Both of them are neutered.

Meh! Ben humps Lilly sometimes. She usually gives him serious bat and he tries again. We generally laugh at it because she is so much stronger then him and he is taking his life into his hands! LOL! Men!

there will be the dominance humping but not like this

Naw thats just sex.

Agree, probably a girl in heat somewhere near by and his hormones are raging.

Dogs also pant and/or become restless when they are in pain. He could be hurting or he could be anxious about somthing. A general vet check often won't spot every little ache or pain, and sometimes there are things going on (in addition to smelling a dog in heat) that dogs are aware of, which we are not.