iPhone randomly beeping and vibrating

I have an iPhone SE running iOS 13.3 and I was setting alarms for tomorrow morning and listening to music through my phone when I noticed a timer beeping sound in the background. so I paused the song and it kept going. there was nothing to suggest the sound was playing except the volume slider kept rising and every time I turned the volume down it would go back up again. I turned off al the alarms thinking maybe one of them was going off somehow but the sound kept going. it was three beeps, a pause, three beeps, so on and so forth. then my phone started vibrating, the long vibration that’s standard for alarms. however the vibrations started getting shorter and more rapid until they lasted little more than a second and were constant. so I restarted my phone and it stopped. there were no notifications in the notification centre and the apps I have notifs on for are very limited and never do that. any ideas?