I took my car to walmart in feb 2010 to have an oil change. then again in nov 2010. they said it did not have any oil in it. i asked was it leaking they said no. they wanted me to sign a waiver so that they could do the oil change. today i went about five miles and my car was making a clanking sound under the hood. the temp gauge went to hot then back down to the middle and then back to hot. i made it home. is my engine shot? I have a 2002 nissan altima.