Coolant low after a hard drive?

[ASpec818]

Ever since replacing my water pump, I've been having coolant level issues. The coolent level was low when I picked the car up from the mechanic, so I refilled about 1/2 gallon distilled water. There didn't seem to be any leaks as my garage is clean. I drove the car about 40 miles today in stop and go traffic, and the coolant light came on again. The level was under the "min" mark. Is it normal to refill the coolant a couple of extra times after it was "bled"? Or does it sound like a problem elsewhere? The mechanic said the hoses were in good shape..