What Could Cause Big Bruise On Thigh?

the other day my left thigh itched really bad and I walked around the swtore scratching it...when I got home I looked and i have a big bruise on my thigh. It hasn't itched since then but it is growing and is sore to the touch....just wondering if I should be concerned and have a dr look at it....It doesn't look like a spider bite or anything and I don't bruise easily so an injury bad enough to leave that kind of mark I would remember.