Before you take your temperature with a thermometer after taking a hot shower or warm bath you should wait at least 1 hour to allow your body to cool down to your normal body temperature.
If you take your temperature before the hour has passed then your body may still be too warm and give you a false reading on the thermometer.
Also it's a good idea to clean the thermometer before you use it to take your temperature using some cool soapy water or even rubbing alcohol.
Also if you smoke or drink hot or cold liquids then wait at least 30 minutes before taking your temperature to allow your body to adjust to the correct temperature.