Doggy Advice Please!!!

My FH and I adopted a Chihuahua/Mini Pinscher mix in October.

He is a year and a half. He is very playful and sweet and loving. He has never been aggressive or biten anyone.

He is alone for most of the day (8am - 6pm), although we have a dog walker come in to walk and play with him in the middle of the day. Everything was fine until last week.

We went to Vegas for Thanksgiving (we live in California) and we bought Spike with us.
Well ever since Spike came back from Vegas he is a changed dog.

He ran out and attacked my neighbor on Monday evening. On Tuesday evening, he climbed on FH's lap and bit him as well.

Spike is fine when I am alone with him, but as soon as FH comes home he start growling and goes into his cage and won't come out.

I know FH didn't do anything to Spike, but it is making me nervous.

Can anyone explain why a dog would change his personality so suddenly?

Yes, he was neutered the first week we adopted him. He was a little angry after that but he was over it in 2 days.

It is a bit bizarre, but my dog does the same with me and FH. All day Sarge is great, but when my FH comes home, the dog starts to get aggressive with ME. It sounds like it is some kind of Alpha male thing that he is expressing. I would talk to the vet about it. I am starting my dog in classes on Tuesday after talking to the vet. I found someone who is really inexpensive. If you live in Suffolk I can give you her information if you would like