How common are dogs to have colitis after surgery?

If there is a two month span between the two, then no, they are not related. Colitis actually means "inflammation of the colon" and wouldn't be related to a hernia surgery anyway. There are lots of things that could bring on colitis...stress, new food/treat, got into something she shouldn't have.

More than likely, your vet prescribed some meds for her...give them to her as prescribed and it should resolve fairly quickly.

Good luck

It can be caused by the antibiotics and pain meds that are often prescribed after surgery, but she would of already been sick. I would think that it is just a coincidence. Colitis has many causes, and it could be secondary to surgery but it has been too long since the surgery for that to be the root of the problem.