Hi.. when I was 20 I was starting to feel lightheaded and dizzy and I would get sick to my stomach and I always felt like I wanted to take a nap. Well I went to the doctor and he told me that I had been hyperventilating. Taking too many deep breaths. I was getting to much oxygen. He just told me to breath in a bag and it worked! This also will cause anxiety if you don't take care of it. He also asked me if I used to smoke and I had just quit about 4 months before that. He told me that because I used to smoke I was still trying to breath like I was inhaling a cigarette. So try breathing in a bag.. It helps me everytime!
I was reading about this and it turns out that breathing into a bag doesn't actually do anything about hyperventliation. What the bag does instead is that it gives you something to focus on while you are breathing and you can get yourself refocused in on breathing properly instead. So if you don't have a bag handy and you're hyperventilating, it helps too.