XR Battery Capacity Depleted by 3% in a Month

I recently bought an iPhone XR from Amazon Renewed. The battery was at 100% when I got it, which fell to 99% less than 24 hours later- not a big deal, i thought. I had started fast charging the phone (wasn’t fast charging when it fell to 99), and after a month it went down to 98% which I assume is normal, considering I usually use a quick charger.

Problem is- the phone went from 98% to 97% within the same week. I’ve also began to notice that the phone is dying quite fast in short amounts of time. This is slightly alarming to me, and I’m wondering if I should send my phone back for a refund, since I’m within the 90-day return period. As far as I know- the phone is completely OEM, but I don’t know if the battery might have been replaced with a bad one or not.

I plan on just getting a XS or 11 Pro when i return but i don’t want to get rid of my phone if not necessary.