Sounds like dhobi rash to me.
Try altering your dhobi powder and the other things already mentioned. I wouldnt shave your arse as the friction after could cause ingrowing hairs and more problems. But a good trim could be helpful. If you have a rash then try some hydrocortisone cream from the chemist and apply twice daily, then come off it slowly (not the tube the cream

) as you lower the dose the body ups its production of cortisone and the rash will stay away.
If it is friction from phys and your abit lardy then it will ease as your arse shrinks. If your a racing snake then do you have runners sphincter when you run? Do you feel the need to shit after a few mins running? If so shit, because clenching, without even knowing could be causing the problem.
Check your washing powder, shower gel, have regular baths with TCP, run with unclenched cheeks try the lycra shorts but wash them after each run, or................................go swimming!