Take a deep breath and calm down. A PET scan doe snot hurt at all to take and is very reassuring for melenoma. I've been working as my mom's care giver for over 2 years on melenoma at the highest level. Meaning that it is as bad as it gets. So .. you are very lucky at this point. The surgery you had has removed all of the melenoma that can be "found" at this time. The PET scan will confirm that.
Should the PET scan or any other test or evern if another place comes up on your skin - it is just that .. another place that can be addressed. With your stage there is no reason for further treatements, like radiation or other chemo options.
The most important thing we have leart is to keep a close watch. Always and forever so that you always catch anything early. So PET SCANS and CAT SCANS and seeing a dermotologist that checks your whole body are all things you should be doing on a regular basis. As your doc how frequently. I'm sure it will start out monthly and work up to 3 months or 6 month intervals. With Mel you never can predict but you can be an active participant in anything it deals out and even in bad cases you can keep a great outlook on life and good quality of life. So far mom has not stopped doing anything she did before and she is at the most advanced level.
Keep the watch on but don't let the "fear" of what could be stop you from lviing your life today.