Many people without PKU report having headaches after consuming aspartame. If you are over 50 and don’t know whether you have PKU, I’d say that if you are not suffering from mental disability, then you do not have PKU. I would suspect that your headache may have been due to the aspartame, but probably not the phenyalanine. My suggestion would be to avoid things with aspartame, which means checking anything labeled as sugar-free for that ingredient. While there are no scientific studies showing that aspartame causes significant problems for anyone without PKU, if you find that it gives you headaches, avoiding it is a reasonable step, just as you’d avoid any food that just doesn’t agree with you.