PKU is detected through a blood test called the Newborn Screening that is usually done shortly after birth. If PKU is not diagnosed and treated shortly after birth in most cases it can cause seizures and eventually mental retardation.
In my opinion (although I am not a doctor, so if you are still concerned please see a doctor) if you have not experienced other issues such as severe learning difficulties, behavior problems, trouble concentrating, etc you probably do not have PKU. I have heard people complain that aspartame gives them headaches, so it isn't that uncommon.
I hope this helps! Let me know if you have other questions.