You need to get a cushion for whichever chair your sitting in. Getting something for your footrest (like another chair, for example) is also imperative.I'm eighteen, I should not be having back pains for no reason, I'm not eighty darn it.
However, even if you just stretch your legs forward and cross them you'll be making a huge difference when it comes to ergonomics. Moving your chair back once in while (or just moving your desk back) so that you have leg room is a good way to do this.
Also, while I never do this myself, everyone needs to sit up straight with their shoulders back. But no one does that anyway, so...
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The reason I'm saying all this to you is because same thing happened to me around the same time during my life as well.
This is why I mostly lie down while playing games, or while online.
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There's also the option of sitting up against the headrest of your bed (it's kinda 'awful', but really comfortable when you have to sit up) whenever you need to change spots.
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