Could it be something serious?

So yesterday my roommates cat threw up and peed on the floor. She stepped over it and just went to work and doesn't seem worried. As far as the pee goes she only cleans his litter box once a month so maybe he just didn't want to use it because it's dirty?

And the vomit wasn't a lot, it looks like it was the food he ate earlier that day

Am I overreacting or should I keep an eye on him?