I hv been taking seretide for about 7 yrs & hv gained about 15kg in these 7yrs, could my medication be the problem of my weight gain?

Unlikely: Although cortisone can induce weight gain, the dose absorbed from seretide is unlikely to be the reason. Has your asthma become control? if not, restricted activities may be part of the reason.

May be: You don't mention what dose of Seretide (Advair in the USA) yo are taking. The fluticasone component of the inhaler has been rarely associated with systemic effects. Also, I assume that between ages 14 and now, you have also grown in height. If not, then reduced activity could also be another cause of the weight gain, as suggested by my colleague.