I still have severe pain weeks after cavity fillings. What can I do?

See your dentist. "severe" pain sounds like you're having pain throughout the day and possibly even having trouble sleeping/eating...If this is the case see your dentist asap.
Tell your dentist. I recommend telling the dentist who treated you- they may recommend using tylenol (acetaminophen) or nsaids. Severe, ongoing pain may indicate an infection.