I get (all year) lightheadedness, dizziness & nasal congestion. Is it likely allergies or due to the pituitary tumor I have? Moving my head worsens it

Pituitary . These are not classic symptoms of most pituitary tumors. Since your dizziness is affected by head movements, an inner ear cause is possible. The nasal congestion may or may not contribute to this by blockage of the eustachian tube which connects the back of the nose to the middle ear. An Otolaryngologist is best to evaluate and advise you.