iPhone XS Max battery

I have been using this phone for a month. At first, the battery remain 100% til morning after I remove the lightning cable. Right now, the battery dropped 8-10% even if I didn't use it. It just happened suddenly. I didn't install any new app recently, which might affect the battery life, or play games. I was wondering is it the battery having problem? It is just one month old iPhone, I felt that it is abnormal. Can I get any advice for this?