Oil in antifreeze?

Okay, so my water pump went bad and was leaking, so I was just running water through it. Then as I was driving one day the belt came off, so I drove home and parked it and it still had water in the engine I'm assuming. Well, it got really cold for a day or two. About a week after that I got out there and changed the water pump and put a new belt on and filled it with the antifreeze drove down the road a few miles it ran good came home check the reservoir and there was oil in it. I ran some K-seal through the coolant system like it said and there is oil and antifreeze mixed coming from somewhere once pressure builds up in the motor. I haven't done anything else to it yet. I was wondering if it could possibly be something not as bad as a cracked block or something.