Brief Answer:
Many possibilities, usually benign and transitory

Detailed Answer:
Hello and welcome to HealthcareMagic.

That feeling of having a heavy head is not that specific, meaning it can have many different causes. One possibility would be tension type headache. It's the most common type of headache affecting more than half of the population at some time or the other, often triggered by stress and anxiety.
Another common possibility would be that of a viral infection like flu which commonly manifest with headache and heavy headed feeling.
Neck arthritis is another common possibility which often is associated with a feeling of heavy head and neck stiffness.
Blood pressure alterations, both high or low blood pressure are also possible causes.

Again as I told you it is commonly encountered and it can have many different causes, I could go on and on listing other possibilities but I don't want to confuse you, in the overwhelming part of cases it's benign and transitory.
What is important is to recognize any indication of something serious like a brain lesion. If this is the only symptom I don't think there is any reason to panic, just rest, stay well hydrated and perhaps take an over the counter anti-inflammatory drug like Ibuprofen which would help for several of those causes.
To be cautious a blood pressure measurement would be wise. Symptoms to make one worry and seek medical attention would be intense headache and vomiting, double vision or vision loss, speech issues, weakness of the limbs on one side, prolonged fever, high blood pressure levels.

I remain at your disposal for other questions.

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