Depressed/Scared rat?

Question
My girlfriend and I have had our rat for about two weeks now. We bought him (Lloyd) from a local pet store, he was a feeder rat. Things were going great until yesterday, Lloyd seemed very happy and curious about everything. He would often get very hyper and jolt across the bed and love to play.

However, the other day we introduced him to one of our cats and Lloyd seemed a bit frightened. The cat didn't try to attack him or anything, the cat was very uninterested. Lloyd hasn't had an encounter with a cat before up until now (that we know of).

Since then Lloyd has been very weird. There has been a huge decrease in his locomotion. He hasn't been in a playful mood (no more jolting around the room). If we take him out of his cage and put him down the first thing he does is run to us, climb up our shirt, and hide behind our neck. He never used to do this. He also lets us hold him and he doesn't move at all. He used to squirm out (no squeaking) because he seemed to want to run around and explore, but now he seems very uninterested.

He is eating fine, he hasn't been squeaking or making noise (no sneezing or anything like that). I've looked everywhere for some sort of help but have been very unsuccessful. The only things I can think of are:
1.He's depressed because he is the only rat in the cage (but we give him more than a couple hours of attention a day)
2.He's sick. Although I see no other symptoms.
3.He's shell shocked from the encounter with the cat. (although the cat was very uninterested in him and from what I know he hasn't had an encounter with a cat before)

Any help would be greatly appreciated!

Thank You,
Andrew & Alison

Answer
He may be stressed. Despite that your cat didn't do anything to him, rats are by nature a prey animal and understand and recognize the smell of a predator. Being alone doesn't help this feeling of fear; as he has no rat buddy to stand around the water cooler and complain with. With a buddy, he'd probably wind up being more outgoing again -- the old addage, safety in numbers. A pair of rats is healthier and happier than a singleton any day, anyway!