I have a mole on my lower back that is usually skin colored and minimally raised. About a week before I start my period it becomes sore, itchy, raised and changes to many different colors. It is currently very sore and red, purple and black in color. When this first started happening (about four years ago), I saw a regular doctor and he said that I probably pinched that area somehow. I have not been back to a doctor since then but this mole is getting larger and more irritating.
See a good dermatologist immediately. A cancerous mole is very easy and painless to get rid of if you catch it early. Skin cancer runs in my family so I see my doctor every six months and he removes anything that looks suspicious. The procedure is incredibly easy, only takes a few minutes and doesn't hurt a bit after it's over (unless you poke the spot). Trust me; I'm the biggest baby in the world when it comes to pain and suffering. If I say it's ok, it really is. The longer a bad mole lingers on, the more complicated your situation becomes. Get rid of it right away! Good luck!