Experiencing additional symptoms along with this lump, will help giving some suggestions for possible diagnosis.
It could be an external hemorrhoid, perianal haematoma (blood blister due to broken blood vessel when the anal sphincter is stretched too much, constipation, straining) or perianal abscess.
Perianal cyst or cancer must be taken in consideration, too.
Physical examination of this area will help your physician making a correct diagnosis.

I dont have any further symptom. Its a small (and I think shrinking) hard lump near the anal sphincter, I can move it around a little bit. it just showed up a week ago. Its mildly annoying.