Battery health on iphone 11

Got iphone from last year january . First everything was going okay dropped 6% in a year . Last time i checked battery health one month ago it was 94 % . today i check it and it's already 91 . dropped 3 percent in a month + sometimes when i play games i see battery going up by itself without charging . one moment I am playing game it prompts up that battery is below 20% i press close then after some time it goes up and down prompting same notification . What to do ?