apple watch connected to charger and get heat

Hi

Follow the instructions here as appropriate, including contacting Apple Support (mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider if necessary:

• Check your battery and charge your Apple Watch - Apple Support
• If your Apple Watch won't charge or it won't turn on - Apple Support
• Keep Apple Watch within acceptable operating temperatures - Apple Support

Be sure to use only an Apple or MFi-certified USB power adapter or third-party accessory in conjunction with the Magnetic Charging Cable or Magnetic Charging Dock when charging your Apple Watch. If you are already doing so, try using another such cable and/or power adapter.

If excessively warm/hot that indicate some sort of fault.

You will need to see about getting a repair. Contact Apple or an authorised repairer.