What is wrong with my cat?

I have a long haired Maine Coon named Pooh Bear. Ive had him for 3 years now and hes been a house cat ever since. I do not know his official age because I took him in after sleeping couple nights at my front door. Had no collar, and appeared to be abandoned by previous owner. Anyways for 4 days now he is acting weird. Slow and sluggish, the cat acts as if he is tremendously weak. his reflexes arent as sharp and the cat is sleeping in a corner of a room upstairs at night. During night time the cat would usually perch up at the windows in the living room. Pooh Bear is also throwing up consistently throughout the day. Nothing comes out, nothing like digested food its just clear fluid like water. His food consumption consists of 1 pouch of gravy like substance with meat in the mornings and crunchy meow mix throughout the day. (I just fill up his bowl with meow mix whenever its empty) He is not eating like he used to. What could be wrong. I have 1 other person living at home including myself thats sick with phlegm and strep throat, could be a fever coming from the weather. Could my cat catch our sickness? is he just sick or getting old possibly dying? Anyway let me know. Thanks