u/Ancient_Stranger3210

So, I am still young but I am way weaker than others ny age and younger. I can barely carry my backpack while others can carry so much more. And I'm not malnourished or anything. I eat enough and healthy food, sleep (i have a really strict sleep schedule) and stay hydrated. I even do some sport, like swimming. But I feel like even walking and carrying myself is heavy. I feel like my bones are so weak. I easily get lightheaded and I always need to sit down or lean against a wall. I also have terrible stamina. And when I do yoga I am so stiff and constantly am off balance. Anyone know what this could be?