STM32F103 PC13 variable duty cycle waveform

I'm intending to use PC13 as an output to drive the onboard LED, but at some point in my program the PC13 pin begins outputting a 4.4 khz signal with pulse width that varies from 0% to 100% back to 0% and keeps repeating that pattern at approx 4 Hz. Is this caused by the RTC calibration function? How can I turn it off. I didn't intentionally turn it on.