I suspect you are kidding yourself, the serial monitor isn't receiving from Bluetooth, and it isn't working at all. Use a proper terminal programme like RealTerm for Communication via Bluetooth. There are several terminals available, all are freebies. I think Groundfungus is right, you have Bluetooth on hardware serial and sharing with the serial monitor, hence the confusion, but no damage done. In real life, it is quite OK to have Bluetooth on hardware serial, but not in a situation like this, so use software serial with Bluetooth on a different pair of pins.