Have been getting used to my tenor over the last couple of months and have been quite happy producing the full range of notes on it when suddenly last night when I tried to play low B or Bb the sound jumps up an octave. Its seems to be consistent on those two notes, however I previously had no problem with theses low notes and could blast them out. Have tried various strength reeds to no avail, nothing improves this. Any ideas what might have caused this