I am pained when foriegners call our sanatan dharma as pagan religion ,why my heart paining what to apply?

The word pagan means 'rural' in earlier days small village communties had their local deities in most cases they were representatives of the forces of nature. At the advent of Christianity in Europe, the message of Christ was spread the Christian missionaries in the countryside but some rural communities refused to accept the new religion. Such rural communities were looked down upon by the Church and the adherents of such beliefs and rituals were named "pagans" that came to be a dirty word over the years. But pagan in itself is a harmless term that means "rural". So you should not feel pained if you are called 'rural'.