Brief Answer:
Lipoma / Cyst as the most probable diagnosis, needs a definitive procedure

Detailed Answer:
Hi.
Thanks for your query.
Noted the history of a hard and not-painful knot between the scrotum and anus.

May I please know some details?
- Since when is the swelling present ?
- Is there any redness ?
- Pain on 'pressure' ? (you have written it as not painful- noted).
- What is the size ?

The probable reasons for painless hard swelling in this area is either a sebaceous cyst or a lipoma, although this is the rare site for these.
Other rare possibilities are always there and can be diagnosed on clinical actual examination, investigations or on removal.

Since this is a rare site for any swelling to occur, I would advise you to get a clinical evaluation and actual examination done preferable by a General Surgeon.
This will give the clinical diagnosis and to undergo the investigations as per his findings.
The best way is to get this removed for a few most important reasons.
1. This is removed and you get rid of a disease for ever.
2. You get a definite diagnosis by histo-pathology examination.

I hope this answer helps you to get a proper diagnosis, and get a proper further treatment. Please feel free to ask for more or if you need further clarifications or if you feel there is a gap of communication.

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Brief Answer:
Per rectal examination and an MRI ASAP

Detailed Answer:
Thanks for your feedback.
The explanation indicates that you should be examined by a Surgeon and may be you need to undergo preferably an MRI of the area. This will confirm the diagnosis.

You may please rate this answer before closure of the discussion and can certainly contact me on the bellow link in future, whenever you may need me.
http://bit.ly/askdrtchandrakant

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