I get that..its anaemia, and poss low blood sugar and pressure. U sound like you are lacking in vitamins and energy nad sound anaemic. Ask your doc for a blood test testing for Anaemia (blood count), folate, vitamin B12 , magnesim and others.
Try asking your doc for Forceval prescription vitamin capsules, i take them and they are very good. You can also take Pharmaton vitamin capsules with ginseng available from chemists/boots etc at around �8.99. They are very energising. You can also take just ginseng with your Forecval capsules. Also, drink plenty of water and eat a high carb meal for energy about an hour before your next spinning class or before any aerobic workout and get an early night the night before your class.