I too thought maybe it was the weather, but yesterday I did a ride near the coast where it didn't get particularly hot (well, at least not compared to inland), with very little climbing, and I was still whupped. Ask MP, she was there to see me constantly fall back.
I've either been overtraining, or I'm coming down with something (my throat does feel a little scratchy), or a little of both. Either way, a break is in order, methinks.