iPhone unexpected shutdowns

My phone has experienced an unexpected shutdown a few months ago, maybe in January of this year. Peak performance has been turned on, but since I think end of feb/start of March it keeps shutting off at random times, especially when I use mobile data. It would shut down, turn on and be on 10% battery then turn off with the no battery icon, if I leave it for a bit or put it on charge and turn it on the battery % is back to what it was before the shutdown. It has happened 6 times now and I’m getting worried. It’s an iPhone SE on the latest update, battery was changed in October 2018 by an apple authorised reseller. Phone was bought in September 2016. 

I take really good care of my phone, never drop it and it has no scratches.