Strange repeated vibrations.

I purchased my iPhone 8 a couple months ago, just recently I updated the OS to IOS 11.1.1, and my phone began to vibrate repeatedly, a couple of times in a row before vibrating again a couple minutes later... I checked, and there were no such notifications from any of my apps to indicate or allow my iPhone to act this way/send vibration notifications. I just want to find out whether this is something normal or something I should be concerned of.

I appreciate any form of info or help.