Skin mole. Melanoma doesn't usually hurt or cause symptoms. Sometimes it itches or bleeds easily. If it is very large it can cause pain. If it has spread to other organs it can also cause symptoms. But if it is small and just on the back, it doesn't cause much in the line of symptoms. Visit your doctor to talk about your options to determine if it really is melanoma (punch biopsy or shave biopsy).
May be no symptoms. While a classic finding is a black, bleeding mole, that isn't always the case. Melanomas can sometimes be pinkish (amelanotic melanoma), rather than brown or black. Best to have someone look over your back regularly, keeping the abcde's of melanoma in mind -- assymetry, irregular border, color (often more than one color in same lesion), diameter (>6mm), and evolving.
Malignant Melanoma. Aad malignant melanoma criteria: asymmetry: 1 half is unlike the other. Border: irregular, scalloped or poorly defined. Color: varied with shades of tan, brown or black; or white, red or blue. Diameter: > 6 mm. Evolving: mole /skin lesion looks different from the rest or is changing in size, shape or color. An irregular mole that itches, bleeds or is painful is worrisome for possible cancer.