My Vivo 4 suddenly starts vibrating with Lv.1 flashing on the screen. I have restarted my phone and reset the device but after a couple of hours it started up again. Sometimes I also get Lv.6 flashing. What could the problem be?

My Vivo 4 suddenly starts vibrating with Lv.1 flashing on the screen. I have restarted my phone and reset the device but after a couple of hours it started up again. Sometimes I also get Lv.6 flashing. What could the problem be?