Hi,
How are you? Are there other symptoms present? If there is no previous history of head trauma, this could be due to primary headaches such as migraine or tension headaches. Migraines usually present with one sided throbbing headaches and may also be associated with nausea, vomiting, and sensitivity to light and sound. Rest, ice pack, avoidance of headache triggers and pain medications may hep provide relief. On the other hand, tension headaches may present with pressure and is often associated with stress. If it persists, have this checked by your doctor for proper evaluation. Take care and regards.
With what you have described, there is no way to safely even guess.
Most headaches are not serious. Ultimately, if you are concerned, you need to see a doctor. How "serious" it is depends on how long you've had the pain, how bad it is, what you have tried that hasn't worked, etc. A doctor will determine if it is a normal headache or if you should have tests done.