2 possibilities here:
- your battery is dead, pretending to be charged at 100% but it's wrong.
- something is draining the battery power, and is forcibly producing heat (CPU, GPU, failing component, ...).
Are the fans spinning at high speed? If yes, something is intensively using the CPU or GPU.
If not, can you locate the area of the mbp that is getting hot? Opening its case would help, but take care not to make any shortcut.