We rescued a dog in February. He's been perfectly fine. Gets along well with our other dogs, gets along with other dogs, etc. He's generally a chilled and happy dog. We got kittens two weeks ago and he was fine with them even. He'd stand there wagging his tail while they played around him.

However since late last week his demeanour has completely changed. He hides behind the couches now, he walks around the house with his wars down and his tail between his legs. He doesn't want to go outside and is constantly cowering in fear. He just wants to lie in his bed or on the couch. Tonight when we let them out he even tried to hide under the car. I have no idea where this behaviour comes from suddenly. If he acted strange around the kittens initially I would have thought it was them but he was fine initially.

I'm waiting to get paid then I'm going to take him to the vet in case it's something physical. Does anyone have any ideas?

EDIT: He's six years old apparently. I have a couple of theories but will still take him to the vet. He isn't suffering from loss of appetite and he seems fine during the day. Today he even played quite nicely with out other dog (he's quite a lazy dog).

1) He's developing a noise phobia (he's constantly listening for noise outside).

2) As mentioned by commenters, maybe he got swiped by one of the kittens (he's a larg dog though so I mean they shouldn't have done so much damage.

3) I had a half day job but left that to work from home mid July. Since Wednesday though I've had to go into the office, and I'm wondering if he's not suffering now from anxiety because of the change of routine. I will be going back to working from home next week, we're just busy with a project that requires my presence in office.