Hi,
I think it's a great time for a vet check up on the kittens. The fact that they are so young and you don't know what happened to the mama (could have died from labor or a infection that could have been passed down to the kittens)
It could very well be a strain from playing with the other kitten, or something more serious. Make sure she is eating and drinking properly as kittens this young get dehydrated fairly quickly. Also watch her stools for any abnormality's such as blood or tarry stool.
At this point the kittens being that they were abandoned need to have a check up and the one a full examination of the neck and shoulder muscles.
Walter
If you are satisfied with my answer please click the *accept button* so I receive Credit for my answer.
**Please consider leaving feedback about me!**