My Mum had a similar problem in one of her feet - her GP, wirthout examining her, repeatedly dismissed it as "feature of growing old" (she was in her 80s). On getting a new GP some years later he examined her and diagnosed the probable cause straight away and sent her for a confirmation angiogram. This found a restricted artery for which they carried out an angioplast immediately - the problem disappeared for the rest of her life.
So, to answer your question, see a good GP (not many around unfortunately, but be persistent).