It's possible you accidentally switched the transmitter from Mode II to Mode I. To change the stick mode, hold down the two "MODE" buttons on the transmitter while you turn it on. You'll then see a display that has "STCK" plus a number. Likely, that number is "1" at this time. Change it to "2", and then turn off the transmitter. Your now back to your regular setup.

This is described on page 5 of the transmitter's instruction manual.