One of my upper molars just started becoming sensitive whenever I drink something cold. The pain pretty much goes away right away. I will be making a dentist appointment in the next week, but is there a good chance I probably have a cavity or something worse?

You probably have some root surface exposure from a slightly receeding gum line. Sometimes all it takes is using a concentrated fluoride gel along with a desentisizing toothpaste. You can also have a sinus inflammation. Upper molars have their roots imbedded in the maxillary sinus. A sinus infection, cold or other inflammation can make the upper molars sensitive. Take some OTC Sudafed and see if the sensitivity goes away while the Sudafed is working. If so, then you're probably dealing with a sinus problem. Be sure to tell your dentist that you tried the Sudafed, even if it doesn't help the sensitivity. Your dentist may also recommend placing a bonded composite over the ares to cover the exposed root surface (if that's the problem). Usually a sensitivity to cold alone that goes away right away is not a sign of something serious. It does need to be checked out, though.