I do not have any problem but i got low blood pressure im 25yrs old girl what would be the reason.?

May be normal: Many young women have low blood pressures, with systolic in 90s not uncommon, and 80s not unheard of. As long as not lightheaded / weak / dizzy, should not be a problem. If any other symptoms, should discuss with your dr in case is a sign of thyroid or other condition.

Depends: Not sure what you mean by low blood pressure. Lower range of normal for most adults is 90/60. However this is not an absolute number, i.e. If ones readings are below this it is necessarily abnormal. Lower blood pressures may be considered normal if you have no symptoms or signs of poor/low circulation. Discuss with you doctor.