You sprayed the pyrethrin directly on the plant? If you did, if it survives, that is the cause. Cannabis cannot take much direct contact with pyrethrins. Yah it will decompose in a few weeks. I suggest you read my spider mite thread, get some neem, and cross your fingers. Pyrethrins should never be put directly on the plant, it will damage them severely.

Edit: Oh ya, don't forget to sterilize the room too.