A question about my cat?

My cat hasnt been feeling well for the past 3 days and i want to know why. 4 days ago my cat was walking around normally for a cat of his age (19 years old) and eating and drinking normally. On monday he i noticed him feeling under the wheather. The next day he threw up twice, but was still drinking a lot of water and walking normally when he wanted to go somewhere. Today he hasnt thrown up but hasnt been drinking and is having trouble walking on one of his back legs. Every day something new is happening to him but the things that happened to him the day before are not happening. He has been throuh MANY things. He fought off a German Shepard when he was 15. He got lost many times, once in the middle of winter with record breaking low temperatures, and another time when he was probably 12 he got extremely sick and was hard as a rock, a week later he was fine. In other words he is a trooper so dont tell me to put him down, tell me what is going on.