It takes time to adjust to a new exercise routine. This goes for anyone, if you are just starting to exercise more regularly, you have commenced training for an event or have upped your exercise quota, you need to allow time for the body to adjust and understand this is a normal part of the process in adaptation.
Are you fuelling your body with the energy it needs. This could mean upping your carb intake to replace depleted glycogen stores, it could mean limiting your caffeine intake (tell that to your latte loving ProSport trainers...) but studies DO show that too much caffeine can interfere with sleep patterns with the outcome of chronic tiredness. Ensure your body receives adequate energy dense food (the good energy, not empty kilojoules).
Replacing fluid loss after sweating and exertion (and anyone who has done an Altitude class at ProSport knows this) is crucial for helping tiredness, improving decision making ability and avoiding tiredness and headaches.
Timing Carbs for Post Exercise:
After a tough session, it is important to top up your muscles glycogen (stored carbohydrate) by having a carb based snack or meal as soon as you can post session. It can be something as simple as a banana!
Low iron can cause general tiredness and slow down your recovery time. Always see a GP to test your iron stores before taking iron supplements.
Finally....if none of these help, have a chat with a GP.