First thing, get the tension outta ur mind.. thats the number one cause of u being skinny..
and hmm.. 6kgs and then 15kgs... thats a lotta weight.. around 45-50lbs...
eat everything in moderate amount and have 5/6 meals throughout the day consisting of mainly protein,carbs and healthy fats... i wud suggest u try to gain weight with the regular food, but if u've tried that already and results didnt show up.. then you should add weight gainer supplement or whey protein into your diet.. it'd require patience.. remember , you wanna gain the healthy weight and not the weight that u'd have to open up another thread to ask us how to lose the fat around ur tummy.!!
Funny !!