5 days ago a small infection appeared on inner side of my legs. The infection increased in size (2-3cm diam), dark skin and it bleeds. How to cure it?

Cannot diagnose. You should show this to a professional. These are generally superficial fungal skin infections but if that was the case and you were not vigorously scratching it should not be bleeding. Get this looked at.
Call your doctor. With symptoms this complex, you owe it to yourself to call your doctor to help you make sense out of them.