1999 Isuzu Rodeo Question Head gasket blown

I just got my car back from the mechanic's after having an oxygen sensor replaced and a new muffler installed. I originally took it in because it didn't pass emissiions. Less than one week after, I think I have blown a head gasket. I have been reading about the white smoke on your sight and I also got hot twice. Once in traffic and once on the highway. I refilled the radiator after the first time and thought it was fine. After it got hot the second time I was shocked to find that the radiator seemed to be close to empty. I literally put maybe 130 miles on this since I got it out of the shop. Could this be due to anything involved with the muffler or oxygen sensor repair? I wanted to ask before I call the mechanic on Monday, as I don't want to be a patsy nor do I want to wag a finger if he did nothing wrong.

Thanks in advance!