In reply to Removed UserDeleted bagger:
No reason to do so at all and vanishingly unlikely to be either abnormal or indeed relevant, I'm afraid.
The blood has leaked out of a damaged blood vessel (as CP says) and initially formed a large valises haematoma. Pressing on it disperses the clot into almost no the tissue layers, hence the spread of the discolouration. If infected you will know as it will be throbbing red and very painful; and you will likely feel absolutely awful.
Simple haematomas turn all sorts of interesting colours as they resolve. I fell off the middle tier at Eglwyseg a few years ago and cunningly landed on my backside. I had an arse like a David Attenborough baboon for several weeks - the green/purple phase was particularly fetching, I'm told...
B