Lactic acid forms in the blood supply, if I remember correctly, so I doubt it's that.

The body is just weird sometimes I reckon. Sometimes you can go into something under hydrated and hungry and ride like a god, and other times you think you have everything right and you ride like poo.

All I try and do is hydrate before, during and especially after, and try to be in a good space mentally. Seems to work 80% of the time.