I have to 100% Agree with remar on this case just because it could be something bad or something that a doctor can treat
Recommendation:
ER or Doctor
Can you say exactly where it's at? Is it at the top of the anus or the start of the crack of your rear? Sorry to ask these questions. I'm asking because if it's close to the anus it could be caused from constipation, straining or passing large stool.
I do think it might be a good idea to see your Dr to find out what's going on. You don't want the cut to get infected and your Dr can most likely prescribed medication to keep that from happening.