ABSOLUTELY GET A SECOND OPINION!!!!!!!
My Dad had a gut feeling that something was wrong, and got the brush off from his GP for a long time before the GP finally agreed to refer him back to his cancer doc (he'd had a melanoma removed over 7 years before and was told he was all clear). His cancer doc checked him over for moles, felt for suspicious lymph nodes, and didn't find anything. He pretty much told my Dad it was all in his head. My Dad still insisted on getting a PETscan, so the doctor finally set it up. Imagine the look on the cancer doc's face when he got to break the news to my Dad that not only was his melanoma back, but it was EVERYWHERE in his body!
Melanoma in it's early stages is very treatable if your doctor isn't an idiot! If my Dad had listened to his cancer doc, my Dad would be dead today. Either don't take no for an answer, or get a new doctor! Let us know what happens!