shots make cat tired/drowsy?

I was at the local animal shelter earlier and they had several beautiful cats but one in particular stood out to me. She was a long haired already spayed young/adult cat. The was sitting in her litter box(they had just been cleaned and several were sitting or playing in them) but she looked very....down. I tried to get her attention and she looked at me but would not come to me or get out of her litter box. The woman cleaning the cages said that she had just gotten her first round of shots and that all the cats acted like that after their shots. I had never heard of this before. She had also just come in to the shelter yesterday or the day before. Is it possible that the routine shots could make a cat drowsy or generally uninterested in being petted? I plan to go back tomorrow to see if she is any more active and I am considering maybe getting her but I would hate to bring a cat home who had something wrong with it. Do animals react to shots or is it something else? The woman said that they all acted like that after they get the shot(s) but I am unsure if that is possible or she just didn't want me to pass up the cat. Her eyes looked clear and there was no discharge as far as I could tell but she was uninterested in being petted at all. She did not cringe away from me or anything. She did blink rather slowly(or at least to me it looked slow) and I am not sure if that could be because she was drowsy or sick. I did not inspect her skin or anything. When I saw she was uninterested in sniffing my hand or being petted I left her alone. Any advise on this subject would be appreciated. It has been a while since I have seen a kitten get shots so am not sure if this is normal. Also, is it okay for her to be sitting in her litter box? The woman said that most of the cats were litter trained and she was one who was trained but I did find it odd that she sat in there. The only other thing in the cage was an empty bowl attached to the door and one layer of news paper between the cat and the steel cage. Perhaps that is why she was sitting in there?

She was adopted by a feral rescue group. She was feeling better the next day, hissing and scratching. It took towels and two people to get her out of the cage and into a crate. One person got really deep scratches and was going to the doctor right after they got her loaded up. The group is hoping to tame her but if not then they will release her in a safe area after she has been fixed. They said they would let me know how she was doing. At least she is not still at the shelter(they put down ferals). I hope all goes well.