I think my cat is sick! Can you help?

For about a week now my one year old male cat has seemed to be trying to cough up a hairball. I purchased some GNC hairball formula and have been giving it to him, and yesterday he stopped coughing. From the day I got him he's been a sweet and loving cat. Today I picked him up to move him off of my office chair and he started growling and showing signs of aggression. I put him on the couch and sat with him for a few minutes then pet his head. He was rubbing his face against my hand but not moving his body much. Once he was comfortable I ran my hands over him to see if there was any tender spots that could be causing his growling. When I got to his hind legs he growled again, and again when I touched his stomach. Not two hours ago he was on my lap lying on his back demanding a belly rub (I think he's part dog). At that time I checked his teeth, ears, and paws (which I do at least once a week) and everything is fine there. He's been eating and drinking normally, and using the litter box regularly. He is an indoor/outdoor cat. When the weather is nice I let him out usually around mid-morning, and he comes in and out all day at his leisure, and once dark sets in I keep him in for the night. Any ideas? I can't bring him to the vet this week, after I paid my bills I literally have $2 dollars in my pocket and there are no free vet clinics in my area.