I am 25 have tightness & ache in chest & upper back for 3 months. i have no other symptoms. if it was heart related would it last this long?

Not likely: Although it is unlikely that your symptoms are cardiac related in a 25 year old woman it is not unlikely for cardiac symptoms to persist for three months. It would be reasonable to seek an evaluation by a health care professional both to exclude cardiac etiology but also to help alleviate your discomfort with an appropriate diagnosis.