Head gasket

I just had my water pump, timing belt, drive belts replaced on my 94 Mitsubishi
Galant. When I picked up the car it was running very rough, and there was water coming out of the tailpipe. The mechanic told me that the car needed a "head job". I'm wondering if the mechanic did something very wrong, because when I brought the car in all it needed was a new water pump. Is there any possibility that something could have gone wrong while the water pump was being replaced to cause this new problem. Is there any link between the water pump, timing belt, etc. And the "head"? Now the car is totally undrivevable, because it's shaking, and runs very rough. I need your help and advice. Thank you.