It's your butt.

OK joking aside - I'm letting this post through because I believe the poster is asking a genuine question about the evident rash.

To answer the question I have 2 suspicions.
1. (most probable) it's a heat/sweat rash - try making sure you exercise good hygiene, that you dry the area thoroughly after washing and wear natural fabrics such as cotton next to that area.

2. possible beginning of a pilonidal cyst - typically forms at the top of the butt crack on the spical region and is believed to be caused by an ingrown hair.