nausea in backstroke

hi im a 13 year old (nearly 14) state swimmer and when i am swimming backstroke i am feeling nausea. it has been happening for about a year now and it is quite annoying me because it is affecting my swimming. my coaches think i will grow out of it, but i do know that it is not my breathing or something to od with my inner ear. it could be motion sickness, but i have tried taking ginger tablets and they didnt work. i am now actually feeling sick during the whole training sesson where before it would only happen when i race. i dont know what is happening and why it is happening, please help!! Rachael