Why is my kitten acting like this?

I adopted my new kitten from a shelter. It has all up to date shots, and has been de wormed. This is my first kitten and it has been treated like a princess. The first two days it was running around and playing. It ate the purina cat food (for all ages) right up and drank tons of water. Now she is lethargic and making gagging sounds. I thought it may had been a hair ball that wouldnt come out but nothing comes out. She lays around all day and night and sleeps and wont do anything by herself. When i place her in front of her food she eats. When I put her in the litter box she poops and pees, but other then that she doesnt move. She looks so sad, and im not sure what to do. The shelter where i took her back quickly handed me amoxicillin and told me to give it to her 3 times a day...but it doesnt seem to be working. HELP!