Back in early November I was in a spirited game of tag on the playground and misjudged where I was landing. My butt cheek landed HARD on a pointy thing. The pain was so bad that I almost threw up. An inch or two inboard and things could have been MUCH worse. Much MUCH worse.
Fast forward to 2 1/2 months later and the bruising color and swelling are gone but it's still tender there on one side. It feels deep and like it could be on something close to the pelvis or to the rear of where my leg bone attaches.
Is there a kind of internal muscle bruising that will get better after more than 3 months? Is this something I need to see a doctor about??
I ended up taking anti-inflammatory a few days after for an unrelated procedure, but never iced it or anything.
There are so many factors.... What's the trend? Better, worse, or the same? How old are you? 
I recall a football game played in snow many years ago, where I--still in my 20's--played against a bunch of younger guys. It was great fun, running, sliding, falling. We played til everyone was about exhausted, then went on to other stuff.
Days later, I was sure I had torn or ruptured something. My lower midsection/groin all hurt pretty badly. I went to the doc, he checked me out, found nothing wrong, and told me to take it easy. Even though the "injury" was basically just muscles that had been asked to do too much, it took me over a month to get over that.
With an impact such as you describe, bruising could take that long to heal. I suppose deep vein thrombosis could be a concern. Most will tell you to see a doc, and that is probably the safest course. In your shoes, I probably wouldn't, and just wait for it to heal up. Just stay tuned to what your body is telling you.
Late November or early December I was trying to get a cabinet safe into my basement. It weighs about 300 lbs but I took the door off to make it 200-ish. Two people were at the top and I was at the bottom. The plan was to lay it on its back and slide it down the steps. It had other ideas. The top people lost their grip and the safe fell flat on its back on the landing. The foot of the safe lifted me like a lever by the ribs, shot me backwards into the header beam, and I landed on all fours on the concrete basement floor.
My ribs were a mess for weeks. Big purple and yellow bruise (now gone, but I don't bruise easily) and a contused lump on my belly which is still there. Doc said no broken ribs. It's still a bit tender.
It goes away. You've likely bruised (a polite word for crushed) tissue somewhere deep inside your rump. I would obviously see a doctor if you feel it necessary to make sure you didn't chip a pelvis or damage a hip joint, but butt bruises suck. Sorry to hear you got hurt.
Saw a video of a guy sitting on a swivel chair, he jumps up screaming and wanders off. Took me a minute to realize the 1" diameter shaft below poked through the bottom of the chair from his weight.
I'll take a deep bruise any time! Poke around and see what the Google Doctor has to say.
Thanks guys. I'll follow my gut and keep waiting it out. I'm in my 40s and the pain has been lessening over time. I forget about it until my wife or kids smack me on the butt, really
I was doing some construction demo about 4-5 years ago, kneeing the ground, pulling on a big prybar, it slipped off and I bashed myself hard on the meaty part of the thigh. It still sorta feels like its asleep when I touch it. Some stuff just never does get right I guess.
In reply to gearheadmb :
Sheesh dude! I guess that's the nuts and bolts of entropy
