@akildoank this "after not long unplugging the charger" sounds very much like a bad battery. Since iFixit does not yet have a guide for that, use something like [https://www.youtube.com/watch?v=0hVTrPRrI_0|this video] Replacement batteries are readily available. Do an online search to find a vendor that appeals to you.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.