iphone 8 will not stay charged for long

my phone started acting up after it had been powered off for a week. i’ll charge it to full battery, but soon after i unplug it, it usually dies. although sometimes i’ve streamed music for an hour and the battery never changes. also, a few weeks before it started acting odd, a bright spot appeared on the bottom left of the screen, that’s the most visible on a white background. this phone is really new to me ( under 3 months ). is there anything i can do, or do i just have to buy another?