Possible cracked tooth? Please advise...

I noticed two days ago that my back molar felt different. No pain or anything, it just felt a little sharp. Upon checking it with a mirror, there is a faint gray horizontal line running through it near the cusp at the very back of my mouth. Does this sound like something I need to be concerned about? It doesn't hurt, but I really have not been able to locate much information about what the gray line might mean, and I have been having a very hard time getting a good look at it to see whether something is wrong. It's making me a little panicked, and because it's a long holiday weekend, I have not been able to get in touch with my dentist. This is very frustrating for me, because I have already been to the dentist twice since Dec. 15, and am awaiting placement of a permanent crown. I have been told only to chew on the side of my mouth that is now causing the problem.