Why would a bruise pop up out of nowhere on my thigh?

I was having allergies all day long. Tons of sneezing spells. I was standing in my room about a week ago and sneezed several times and then a horrible pain came in my inner thigh on my right leg. It hurt for a day or so and then a small bruise popped up. Over the course of the next few days it got larger and purple. The bruise hasn't gone away And is radiating up to my groin and down to my knee. It hurts to get in and out of a car and hurts even more to stretch. What could this be? I never hit my leg or anything to cause a bruise nor have any idea why it's not going away.