What Causes Circular Bruise On Thighs?

I am a 22 year old female. I am 5'1" and weigh 114 lbs. About a week ago I noticed a perfectly circular bruise on the inside of my left thigh, about halfway between my knee and my hip. I don't know how I could have gotten it. I don't remember doing anything that would have caused bruising, and I only ever wear jeans or pants, no skirts or shorts. It was originally black and blue in the center and green at the edges, and about an inch in diameter. The shape has not changed at all. A few days later I noticed that the green was changing to black and blue, and in the center was turning red. Now, about half of it, all around the edge, is black and blue. The rest is red with white stripes and swirls and when you touch it you can feel a perfectly flat lump right where the red and white coloring is. The lump was not there originally, and only appeared last night. It started out small, and by this morning was edging out of the red area and into the black and blue. As I said, nothing but my pants, myself, my husband and water has come into contact with that area of my thigh in a several months. Also, I went to the hospital a couple of days ago because I was nauseous and had some pain in my lower right abdomen that spread to my groin when I walked or sat up for too long. They took a urine sample and said I had a UTI and a viral infection, and gave me antibiotics for the former, which I just got filled but haven't started yet.