I can't put a single pound on me. Help?

Im 16, 5.6 short (lol) and weight 60kg. My problem is the fact that I can't increase my muscle mass. I stopped track and field training for summer 2 months ago. I measured my weight about 1 or 2 months ago and I went from 63kg to 60. Since then I've been eating more meals. Small meals 5 to 6 per day. I only eat 1 or 2 shitty fat meals per week. I do light gym exerciceand aerobic running. When I was on vacation I ate protein everyday lol (not only red meat). I weighted yesterday and Im still 60kg. I really dont know what to do. I want to take 5kg (pure muscle) and stick there.
I can't put a single pound on me. Help?
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