What could this coccyx mass be?

[Bubulinu]

Hi,

I'm in my early twenties, and I just had an echo ultrasound done of my coccyx region. It revealed one pilonidal cyst outside of my butt crack to the right, and lower down an unidentified mass, which the doctor doing the echo didn't know what it was. The mass is located just a little below the very top of the butt crack. Where the mass ends going down along the butt crack, there is a sinus dimple opening. Not sure if the two are related. Lower down the butt crack, the end of the coccyx can be palpated, and there is no mass there. The mass can be palpated with fingers, and it is somewhat rubbery but at the same time firm and similar to a bone, and not very movable (although it is somewhat movable, but in a limited manner). This mass does not hurt when pressing on it, neither does it cause me any pain.

I've posted the echo ultrasound picture of this unidentified mass in my profile under the pictures (http://www.medhelp.org/user_photos/show/735516?personal_page_id=3184763). It is 1.6cm long, 0.6cm wide and located at a depth of 0.7cm. The ultrasound also suggests that it is a mass having calcifications, hence accounting for the firm, bone-like feeling. What could this be? I've read that sacrococcygeal chrodoma can manifest as a lump in that region, however, it is often associated with pain in the region and/or changes in bowel movement and bladder function (I have none of those) as well as being extremely rare considering all age groups (1:1,000,000 cases) and especially at my age, in my early 20s, most chordomas occurring in the 40-70 age group.  Any ideas what this could be?

Thanks!