if you knew exactly what they did to you during the operation, you wouldn't be at all surprised that you have bruising and swelling. it's more like carpentry than surgery!
i found my ankles took weeks and weeks, maybe even months, to stop swelling up, and at every opportunity i would sit with the affected leg raised on a stool (i've had both hips resurfaced in separate ops). i also got rid of the compression stocking after a week, and instead, used to walk around for about 10 minutes every hour to keep the circulation going. and while i was sitting i used to do the flexing of the foot exercise, and the drawing a circle with the toes exercise - all designed to keep the circulation in your leg going.
3 weeks post op is very early days, you'll be loads better in another 3 weeks. as bednobs has said though, put your mind at rest and contact your gp if you're really worried.