What could have happened?

I know my one dog has separation anxiety. I work from home so am home most of the time.

On Saturday they were alone for most of the day. About six/ seven hours. They were all fine that evening, Sunday and Monday morning.

Monday afternoon I had to go out again. I was gone for three hours.

When I got back, my one dog was so stressed. Then he didn't want to eat last night. He slept in my husband's cupboard last night instead of on their bed with two of our other dogs (we have four in total).

This morning he was still stressed. Hiding behind the bathroom door, then he went and slept in his kennel outside (which I didn't even realise they still use, as we leave the doors open for them all day and when we're not here).

I had to go to a meeting so I took him to my sister's house to play with her dogs while I was busy. He was happy as a clam there. Playing, running around, wagging tail.

Come back home, and he's stressed again. Ran straight outside into his kennel. Then I called him and locked him inside with us (I wanted to let the birds out). Now he's sleeping in my husband's cupboard again.

He eats snacks. Chicken and cold meat. He's drinking water.

What could have happened that he's so stressed? All our other pets are fine and if anything were to have happened, our pitbull would have been the bigger target.