Red Eared Slider turtle SICK AND WON'T EAT, looks like he's having trouble breathing!

PLEASE COMMENT BELOW WITH YOUR THOUGHTS!! SICK TURTLE!! I've had this baby Red Eared Slider (that I bought at a 99cent store) for about 4 months now. For the first few months, he was doing very well-eating all of his food, getting plenty of exercise, swimming about his cage. Recently, about three weeks ago now, he completely stopped eating. I fed him a diet of turtle pellets, meal worms, shrimp, and occasionally lettuce. He used to eat 5 or 6 meal worms at a time, now he won't eat a single thing. He also seems to be doing something weird with his neck--it looks like he's either having trouble breathing, or is attempting to vomit. PLEASE, IF YOU HAVE ANY IDEA WHAT IS WRONG WITH MY TURTLE, COMMENT BELOW!!