Have you been to a doctor to check your hormone levels? It may be normal for you to have irregular periods, but at 18, you should start getting regular check-ups anyway.

Do the other women in your family have larger breasts or are they on the small side? If they are small breasted, then you probably will be, too.

The exercising should only have an effect if your body fat percentage is very low. It happens with a lot of female athletes, especially if they start young. Look at gymnasts, figure skaters, ballet dancers, etc. They keep their body fat low and they usually have no breasts. Have you always been very fit? Even when you were 12-14 years old?

You should go to the doctor and talk about all of this. Usually breast development is about finished around age 18, but they can fill out a bit after that (mine did).

well my mam has small breasts
i started exercising when i was about 15-16

i havent always been fit, but i have been skinny till got diagnosed for hyperthyroidism since it was the cause of me losing weight due to my metabolism being high.

therefore i had to gain weight and so i have doubled my work out. It's just my body is uneven, got no breast and im quite big.. :S

i prob will call my gp just to know...