I woke up one morning and one of my pupils was dilated, is this serious?

Potentially. It could be secondary to local contact with a medication, trauma or something putting pressure on on of the nerves to the eye. I would have it evaluated without delay.
3rd nerve palsy? You should seek the guidance of an ophthalmologist or neurologist.
It can be. This can be due to certain medical diseases, exposure to certain medications and to certain structural abnormalities inside the head. This needs to be evaluated by your doctor.