I am 20 years old and my breast size is 36.am i developing some disease?

Unlikely: Breast disease at age 20 would be unusual. Size 36 is a normal size for breasts. Every woman is different and have different breast sizes, large, small, and in between.

I Doubt It: Breast size is only a single measurement. Let's say your chest below the breasts is 34". Congrats - you're a b cup. Regardless, if you look proportional, and both breasts are growing together, you're probably fine. Ask your doc at your next ob/gyn appt (you are going for pap smears and your hpv vaccine, right? Right!?).