I have lump on my bum che
Its a blocked and infected hair follicle. They are quite common on the buttocks and always painful when they get to this stage.
You may need to visit your doctor to get antibiotics if it continues to swell,but in the mean time keep it well washed and dont pick at it. You could also try gently lathering it with a very soft brush,but dont scrub or it will bleed.
You may get more of these so continue to either use a brush and lather to loosen dead skin on your buttocks or use an exfoliating wash…if you are in UK look for St Ives Apricot scrub in the supermarket ,which you will find with face washes. At night use a skin cream after washing. Try to do this at least twice per week.