There are some things you should look for in determining whether or not it's skin cancer -
A = Asymmetry
One half is unlike the other half.
B = Border
An irregular, scalloped or poorly defined border.
C = Color
Is varied from one area to another; has shades of tan, brown or black, or is sometimes white, red, or blue.
D = Diameter
Melanomas are usually greater than 6mm (the size of a pencil eraser) when diagnosed, but they can be smaller.
E = Evolving
A mole or skin lesion that looks different from the rest or is changing in size, shape or color.
However, just because it may not be skin cancer (and I have no way of knowing that or not), it doesn't mean it isn't something that needs to be treated. You should definitely have a doctor take a look to be on the safe side.
Let us know what happens.
Have it evaluated by a Dermatologist. Very easy to misread cancer warning signs. I had what I thought was a round red pimple that would not go away and it turned out to be Squamous Cell skin cancer that had to be surgically removed.