Suggest Treatment For Small Purple Line In Crease Of Leg

I noticed a small purple line in the crease of my leg two days ago and it was very painful. Today it is larger than the size of a walnut, deep purple in color and is very tender to the touch. The symptoms are similar to a bruise but I have not bumped that area, nor is it very possible to bump that area, and so I am concerned with what it may be, what I should do and if I need to see someone.