Why is my pseudotumour cerebri causing the back of my neck to be painful and stiff? what causes this?

Increased pressures: Hmmm....from the question it sounds like you're not clear on WHAT PSEUDOTUMOR CEREBRI actually is to start with...so in the first place, it's an older term that has been replaced by IDIOPATHIC INTRACRANIAL HYPERTENSION (IIH). In this condition fluid pressures of the cerebrospinal fluid are highly elevated and therefore squeezing the brain from all directions. When pressures are high..pain ensues