My cat won't leave me alone, and I dont know why!! 10 points read plz cat owners!!?

I am a 14 year old girl. I have a cat, who is a 1 and a half year old male. I got him at 9 months old, from the animal shelter. He has never ever been an affectionate cat since i got him. But 4 days ago, he suddenly changed! Now he won't leave me alone, and I dont know why. He never liked to be stroked... now he follows me around the house ALL the time, for the last 4 days. When I have a shower, he will yowl at the door to be in the bathroom with me!! His meow is so annoying, i give up and let him in lol. When I have a bath, he will sit on the end of the bathtub! When I leave a room, even if he's sleeping, he will follow me out to wherever i go. He even tries to follow me out the door when i leave the house!! (he's an indoor cat). When I go to bed, he will like sit on the end of the bed all night, and if anyone comes near me he will hiss/growl at them lol. Nothing has changed at all in his life. Same food, same toys, same bed, same everything! He was already neutered when i got him. I dont get it! And he has suddenly been demanding attention so much its really starting to annoy me. I love him so much, but theres only so much felix a person can take lol! He walks all over my lap when i'm eating dinner, sits on my paper when i'm doing homework, and meows/scratches the door when i shut him out of a room. The only way i can get away from him, is to throw something and run as fast as i can into a room, so he cant see where i went lol. But then when i come out of the room he looks so sad, like i abandoned him or something, i go give him a hug and he's ok again lol. If he just wanted to be in the same room as me, it would be ok. But he wants to be ON me! I need some time for myself without my cat lool. I spend alot of time caring for him, playing fetch with him hehe and loving him. Before the last 4 days, i spent at least 3 hours a day with him. But suddenly this all happend! He has never been protective of me. When i'm asleep, and someone comes into my room, he will get all wound up. Then, if someone approaches me, he will lunge himself at them lol! Like yesterday morning, my dad came into my room to wake me up, and he shook me, and felix hissed, ran up to him, and leaped onto my dads arm, scratching and biting him lol. My dad had to shake him off! Luckily my dad wasnt too badly scratched, and felix wasn't hurt either. But why has this happend?! will he stop?! HELP! I dont want to take him to the vet unless its completely neccessary, because we dont have that much money to spend at the vet. And i dont think he's ill. HELP!!!!!!!!!!!!!