You just described my four years of college XC.
I didn't put together why it was happening to me until after college. Basically, you're spent. You've either overtrained and now it's getting you, or your coach has decided that "now that championship time is here, we have to cut down the mileage/ramp up the intensity/do some stupid workouts we've never done before/etc" and it's killing you.
For me, what I discovered about myself is that I can only withstand about 6 weeks of the hard interval training that we did all season long. By mid-season, man, I was killin it. By the end of the season...man, I was DEAD. I never ran really high mileage (70-75 most of the season), but the intensity was too high for too long for me. Plus, our coach used to think that we should do all of those things I mentioned above. There's nothing worse than your legs feeling like they're absolutely burning with lactic acid on a 5 mile easy run.
Your specific cause may be different than mine was, but the symptoms are classic overtraining.