Can someone explain to me how come my nose always blocked?

Allergies? Viagra (sildenafil)?: Allergies are probably most common cause of nasal congestion aka stuffy nose. Deviated septum can also do this too. Viagra (sildenafil) & its competitors can increase blood flow to vascular tissues leading to nasal congestion (and other more desired effects). Go see your Family Doc for evaluation, treatment & referral to ENT if indicated.