I have a big bruise on my thigh.I dont know how I got it and it doesnt hurt when I touch it, what should I do?

You bumped it and didn't realise. Do nothing and quit worrying.

Just ignore it. It will go away, I know way too many people that get random bruises that they don't remember getting and doesn't hurt. However if it doesn't go away or weaken (the intensity of th color)for a week or two, just go to the doctor to get it checked just in case.

Many people get bruises all the time, even ones we don't notice. So don't fret; bruises are simply broken capillaries beneath the skin. It is the severity of the bruise and the underlying muscular damage that makes it hurt, and not the appearance of a bruise. Most bruises people usually get hurts because the damage is severe enough to cause tissue damage; this causes the pain receptors to react. Still, you're lucky; my body suffers from anemia (my blood doesn't carry enough oxygen to my body's tissue) so even simple bruises cause me pain. Now, try having that AND take 7 years of martial arts. Anyway, your bruise is nothing serious. But If you want to be sure, see a doctor. Hope this helps! x3