2002 BMW 525 Engine problems

I recently had a breather hose put on, a fuel filter installed and a radiator put on. We also noticed that there was a small crack in the reservoir tank, which caused antifreeze to leak out on the ground. The mechanic repaired the small crack and it appears that there is no more leaking so far. Today when I start my car, I get a puff of smoke coming out of the tail pipe and some smoke coming from underneath the hood. After I pulled off, the smoke stopped, however later the "check the engine coolant level" code came on. Please tell me why the engine code came on and why I'm getting a puff of smoke coming from the tail pipe?