Let's start with this.
Today, iOS 11.3 has been released.
Please connect your iPhone SE to a computer to update it (to keep the power levels while updating).
Now there is a new section in iOS 11.3 under Settings --> Battery.
What does the health of the battery say?
It could be you received a defective battery and should be exchanged.
However this is the Q&A forum, which is a different entity from the store.
If your iOS 11.3 indicates a defective battery, please contact https://help.ifixit.com/