Is this something serious?
Not always. There could be few reasons that a tooth is a sensitive for temperature. One of the reasons could be recent large fillings. With time however these fillings should be less and less sensitive for temperature. If the sensitivity is constant for a long time please see a dentist. If left alone it can get worse and it may end up with a root canal.
The othe reason for sensitivity can be exposed or damaged gums when you brush too hard.
In general I advise to use toothbrushes and toothpastes marked “for sensitive teeth”. Not many know this, but results of using “sensitive” toothpaste and toothbrush will appear after around two weeks.