Thanks for the query to H.C.M. Forum.
The lump ( tumor
) which reoccurs after some ( as you not mention the time).
May be a sebaceous cyst , but not necessary as according to you this is movable ,but possibly not.
As recurrence is there , so this is the cause of concern.
Flexes because pinching of underlying nerve
under this tumor.
In my opinion get in an X ray
and C T SCAN of this part.
Consult an Orthopaedic surgeon
and get his opinion.