Why does my battery health on my iPhone 11 Pro goes down so fast

i buyed my iPhone 11 Pro for a half year ago since then my battery health has going down so fast I charge it normally for one week ago I reached 90% on my battery health and for 4 days ago I reached 89% and today I reached 87% it goes down so fast I use it normal I don’t overcharge it and I have noticed when it reached 90% the battery drained much faster than normal and I have to charge it 3 times on a day I’m worried about my battery what should I do is this something wrong with the battery does this goes on the one year warranty I have not Apple care but since my phone is not 1 year old does this goes on the warranty