Hard stool, mucus like blood, no worms :( ?

Hello!
I have had my cat for 3 months and he is 8 months old. He was a rescue cat and the woman who found him takes in cats gets them fixed, gets their shots and de worms them. for 2 months he has been fine. For 3 separate occasions i have noticed hard stools, little blood and yet no worms or anything. He is a house cat and i check him all the time. I do not know what is going on or is wrong. Help!

What makes you think he has worms?  They don't give cats hard stools.

Any other unusual signs?  Do the hard stools co-incide with a particular food?

I don't think he has worms. No he is always running around very active, and i recently changed the food to adult cat since we also have a 9 yr old cat. Before we had two separate foods for them. His was for kittens and hers was for hair loss. I watch him and he doesn't do anything out or the ordinary and he seems to be fine. Its just awkward to see 3 stools with blood in them. My hubby says he has seen it too. My hubby doesn't think the cat needs to be seen by the vet, but I don't like taking chances.