You're at a good BMI
only takes height
and weight into consideration and so at 5'4" and 9stone3 (129lbs) you have a bmi of 22.
1 normal bmi weight range is 18.5 to 24.5. Online calculator: http://www.nhlbisupport.com/bmi/
bmi is not always the best measurement. This flickr set tries to illustrate that bmi categories can be misleading as well. http://www.flickr.com/photos/77367764@n00/sets/72157602199008819/.
Answer assisted by Chethan Sarabu, Medical Student