Cat is sick?

Warning: Mentions of poop

Came home late last night and my cat pooped on my bed. It was pretty solid so I just cleaned up the mattress with the sheets in the wash and slept in my brother's bed since he's gone for Uni. 

I woke up and when I walked back to my room I saw that he pooped on my mattress! Cleaned as best I can before going to work. It was a solid stool this time too. 

Then my dad texts me that our cat pooped on my brother's bed! This time it was liquid.

We have an automatic litter box that he's been using and it seems clean with nothing blocking its entrance, so he does have normal access to it. I'll take him to the vet ASAP so this problem is no more, but does anyone have an idea of what he may have?