Is your email written by a non-native speaker? I have not come across any native speaker using 'feeling flash' till date.
But, if he is a non-native speaker...
he means he's not feeling energetic. 'Flash' here is probably used to describe the energy within. I myself use it, though very rarely [The reason why I use is I think it's closely related to the 'spark', the energy]. Mind it, it's not a standard usage.
It's not directly related but...I found an entry in the WordWeb Dictionary:
flash (noun, sense #1): A sudden intense burst of radiant energy
The friend is not feeling energetic and thus, as the comment describes, he's not taking part in whatsoever event/sport.
There are rare chances that a non-native speaker might have heard the usage of 'flush' and thus misunderstood it. The pronunciation of 'flash' and 'flush' is somewhat similar. This is because 'flush' is typically used to say that you are 'sick' (hot feeling as in fever). However, in such cases, in fact, you feel flush.