Yes, you should see your GP as soon as possible. The dosage you''re taking should not be followed for longer than about four days without the advice of a doctor due to the risk of liver damage
In the meantime, if you feel there's an unacceptable delay in seeing a GP, it might be best for you to take soluble aspirin or ibuprofen instead as long as you've never suffered any gastric problems in the past. Both aspirin and ibuprofen can be taken for longer periods than paracetamol before any toxic symptoms arise.
to answer your last question, in simple terms, paracetamol can irreversibly "poison" the liver due to its interference with normal liver function.. Once it's done,there's no going back and depending on the damage caused, it may even be lethal.