Why i am fat? i dont know whats the reason for it... i eat nutritious food then also...?

i m 18 yr old grl and my thyroid is below 30.i eat nutritious food 3 times a day......i walk for half an hour..and take 15 mins for cycling. I take proper rest also and wake at the correct timings...but my friends eat more than me... and dey dnt exercise also...but stilll they r slim...i eat in limit and i m fat... but y it is with me only that i m fat and they r not..my eyes r puffy wid dark circles..but why.there is no heredity problem also..my thighs ar very fatty with hanging flab... my legs r also.. small comparatively to my upper body...my thighs show some strech marks... like old people....i drink milk once a day...but my height is 5 foot only...why its wid me only... m i so bad for god or what else.....please give me the reason for my fatty body... my darkcircles under eyes.....plesssssssssssssssssssssssss…