Why do a few bars of my piano sheet music have a treble clef in place of the b clef?

Unless it's a misprint in the music, simply play the notes indicated with your left hand. I'm sure you realize this, but don't just play it like its in b cleff but an octave up. The lines and spaces of b cleff do not represent the same notes as the lines and spaces of treble cleff. Simply put, you will just be playing two layers of treble cleff music.