Cat question, Sick or not?

so my girlfriend and I adopted a cat from a shelter. she is about 8 months to 1 year old and she was so sweet when we got her. We took her to the vet the same week we got her, and they said she had kennel cough and gave us antibiotics to treat it. when we brought her home, she was not very playfull and was not eating very much even when we tried Tuna fish, Chicken, Catnip, and such that every other cat I have ever owned would have come running for. After a few days of the antibiotics, she did start eating and she became more playfull. however, this week, (about 3 weeks after we got her) she has become very lethargic, and sleeps all the time. She has become very Timid and shy and wont leave our bed. She has no problems with us, but she gets scared when you pick her up or try to play with her, the first two weeks we had her, she loved to be held and did start playing. Now she seems to be going backwards. Any Ideas? she is eating, she is not vomitting, and she does not have Diareha...