I don't know your location, but maybe it was just too hot? I've felt somewhat similarly when it's too hot outside and I do too much.
I'm sure someone else more familiar with training and how much is OK will come along to share advice, but as far as anxiety goes, your mind can really play tricks on you! I'm sure you're already aware of that though. Sounds like you're awfully worried about the upcoming ride and its associated pressures.
Good luck!