Brief Answer:
May be an inclusion cyst or a rare chance of being a wart.

Detailed Answer:
Hi.
Thanks for your query and history, albeit small and specific.
May I please know a few more things:
-Since when have you got this lump ? Noticed just over a week or multiplying over last week ( present over a long time but now enlarging) ?
-Is the skin over it normal in color, texture or has changed ?
- Please post a photograph / picture attached to this query if it is possible for you.

The probable cause of such a bump which is movable, and seems to be multiplying over the last week, without pain, slips away on touching and is at the underside of the middle finger can be :
A cyst- this is called either an inclusion cyst or a dermoid cyst (if the skin is intact and appearing normal)
May be a wart (if the skin is broken, I am thinking on this line as you give a history of ''multiplying over'' without the history related to the condition of the skin)

I would advise you the following:
To keep it under observation;
Still better is to get this seen by a Surgeon and follow his advise. This is because the bump is on the finger and if it enlarges more or gets infected, may cause a problem for functioning of the finger.

I hope this answer helps you in proper decision making, please feel to ask for more if you need to or if you feel there is a gap of communication. Awaiting your feedback.
Thanks you and wishing you a very happy new year.

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Brief Answer:
If skin intact - inclusion cyst, if skin involved - wart.

Detailed Answer:
Thanks for your feedback.

No pain- moves- little raising on the outer skin - noted two weeks ago and again yesterday enlargement with smaller sitting like a new growth.

If the skin is intact and normal, this is just an inclusion cyst. You may keep it under observation after a single consultation with a Surgeon.

If the skin is involved - it may be a wart.

Pictures would have helped me.
Rest of the things as already discussed above.

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