Is there a reason why i am having so much pain after having a porcelian onlay?

Nerve: There are many possibilities why you have the onlay treatment. Such as cracked tooth, large caries or failed filling ..., etc. But they pointed to one thing, you have compromised tooth structure and compromised tooth pulp chamber where nerve and blood vessels resides. If you have the pain after the treatment, you should contact your dentist for re-evaluation.