If you start to lack in energy there are two main things I would suggest you do first to try and understand why it is happening:
Keep a food journal
Record how you feel after you have eaten. A food journal should include what you have eaten, the time you ate it, approx. the portion size and if it made you feel sleepy/bloated/sick etc……This could be a way of identifying if a certain food is making you feel lethargic.
Keep a sleep diary
Make sure you are getting enough sleep. Not always possible I know, but eliminating blue light from phones or tablets late at night, reading before bed (a book) and not eating or drinking alcohol just before you go to bed can all help improve the quality of your sleep. I try to make sure I don’t intake any caffeine after 2pm. Caffeine stays active within the body for up to 8 hours, so if your drinking it late at night it will most definitely affect your sleep.
If both of these things don’t bring you any answers try a little bit of gentle exercise regularly. Yoga and walking can help reduce cortisol in the body and hopefully give you a bit more energy.
If you have been training really hard, your body is perhaps just tired. Listening to the body and being kind to it is as important as training to achieve a result. There has to be a balance.
I would definitely recommend finding the root cause of the problem rather than guessing and spending money on supplements or drinking stimulants to combat tiredness.