A 21-year-old unmarried woman has premenstrual fullness of breast and pain, the likely diagnosis is:

It is usually seen in women of 4th and 5th decade of life (also not rare in young females) with complain of an intermittent mammary discomfort or an area of lumpiness or nodularity in the breast. The changes are generally bilateral. Pain is cyclical with a premenstrual exacerbation. Most likely the girl in question is suffering from fibroadenosis though her age is not consistent with the fibroadenosis.