I could be anything from hard exercise, though that is a large bruise, to a blood disorder and everything in between. Are you on any type of blood thinner? How is your diet? I know there are vitamins that, if not enough, you can bruise easier. K is one, and not sure of any others D or C?). Also, anemia can make you bruise easier, though there are usually other signs that point right to that.
Low platelets, too much ibuprofen (or whatever it is called in the UK., and many diseases can cause this, but I am not going to go to worse case scenario, as that does not help. Are there other symptoms? Fatigue, severe pain, nosebleeds, heavier than normal menstrual cycle?
I once did a lot of leg exercises and the next day had bruises around my upper thigh and at the knees. Worried me until the gym trainer told me that I over did it.
Put ice on it for now and don't take any OTC pain relievers like asprin, ibuprofen or Aleve (whatever the ingredient in there is can cause bruising if over-used).
Or call whomever does the over the phone triage if putting ice on it doesn't thrill you. If you do use ice, only 20 minutes at most every few hours.
How big in area is the bruise?