My sense of taste is playing tricks with me, too.
Yesterday, I was smoking an English blend, and it was giving me a very pronounced sour taste right from the get-go. I smoked at least 1/4 of a bowl that way and could taste sour on every puff.
I then let the pipe go out and put it aside for a few minutes. When I picked it up again, the sourness was gone, as if it was never there. Finished the bowl with no problem.
Mind you, I have smoked this blend multiple times before, from that same pouch, and it never tasted sour.