my daughter had something very similar when she was about that age. She said she felt really hot, dizzy, hearing her heartbeat in her ears, feeling sick, and aware of things like people speaking but not taking in what was being said, the colour would also drain from her face. She had all sorts of tests, an MRI scan, an EEG, loads of ECGs she wore a 24 hour heart monitor, eventually, they said that although she had a slightly irregular heartbeat, they didn't feel that it was causing the problem. The only other test available was to have a heart monitor placed under the skin so that when she had one of these episodes, she could record what her heart was doing at the time. As her episodes were not that frequent, we felt that at that time it wasn't the right thing to do by having an invasive procedure. She hasn't had another episode since thankfully, so I guess we will never know what really caused them. Maybe hormonal due to their age. They did also suggest that she may be dehydrated, but she always felt that she was drinking enough.