The sneeze I can see as a way your body was successfully transferring energy. The congestion I would see as a failed transfer of energy. Perhaps the way the gods are telling you not to do the spell or just plain no.
The sneeze and congestion could be entirely unrelated to the casting. What people seem to forget far to often is that not everything that happens has some sort of significant meaning. Some things do, but not everything. The sneeze could have simply meant that some dust was disturbed and got up your nose, congestion could also be the result of that.
Furthermore, and I don't mean to sound rude if I do, but if the best thing beings as powerful as gods and goddesses are supposed to be can come up with to say "no" is a sneeze, they really need to work on their communication skills.