coolant leak

I recently took my vehicle in to a well known auto repair shop because it was running hot while idleing. The business informed me I needed a new thermostat and a coolant system flush. So I had them do the repairs. As soon as I brought the vehicle home I noticed it was leaking coolant. It had never leaked coolant before. I called the busniness and they told me to just drive it around for a few days to make sure it was coolant spilled on the engine burning off. I waited a week. Took the vehicle back, they checked it and they say now I have a leak in one of the freeze plugs that in the engine. They dont do engine repair work. I mentioned to them that the vehicle was not leaking coolant prior to them working on it. I was told that after a thermostat is changed it can cause a weak spot in the engine to start leaking. Is this all true, or is it leaking now because of something they did???