Katrina Evemy died in hospital on Wednesday following the incident on 13 April at Graig Avenue.
A 21-year-old man was charged with attempted murder following the incident, but police now say they are treating it as murder.
Ms Evemy's family are being supported by Dyfed-Powys Police officers.
Senior investigating officer DCI Ifan Charles said: "On behalf of Dyfed-Powys Police I extend deepest sympathies to the family of Katrina Evemy.
"Our specialist officers are continuing to support the family at this extremely difficult time.
"The death of Katrina means we are now investigating a murder. The investigation is on-going and all lines of enquiry are being followed."
A murder investigation has been launched following the death of a 19-year-old woman nearly a week after an incident at her Llanelli home.