Thanks for the query to H.C.M. Forum.
Pain in the finger nail that too from last,
Localized pain at one point,
Diabetic family history,
No swelling ,
No cuts or injury
Pain may be in the bone of finger,
May be due to infection of bone of finger.
As there is history of diabetes
so please don't take it lightly.
Few things are necessary for diagnosis as,X ray
of effected part,
Age is also important.
Blood examination for sugar, E S R ( if very high indicator of some chronic infection).
So consult a physician and get in above mentioned examination so that as early as possible diagnosis could be established.
Hope I have answered your query.
If further any question I will be happy to help.