Hi there - I'm not a doctor, so I can't give you a definite answer, but I can tell you that I've had the same experience before with a couple of very bad, deep bruises, and also a very extreme sunburn. I *think* it happens due to the inflammation and swelling, and then when you stand, the blood rushes down and somehow causes severe pain that goes away after a few minutes. In my case, elevating the leg(s) and using ice packs helped.
Are you still on painkillers? If you're not on prescription pain killers, you might want to try taking an OTC anti-inflammatory like ibuprofen to help relieve the pain. As with anything, if it doesn't get steadily better, or if it worsens, definitely go to the doc.