Hi. I ve gone through your query and I can understand your anxiety. Headaches is one of the common side effect of amoxicillin due to the muscle contraction in shoulders, neck, scalp, and jaw & is less serious. Also if your symptoms could be of amoxicillin-induced aseptic meningitis means, along with the presenting complaints like headache and stiff neck, fever also should be present. This manifests within a week of tablet intake. As Excedrin is a very good option for treating migraines & stiff neck, your symptoms might be due to amoxicillin. I advice you to go to ER department, -to do a detailed clinical examination for meningitis - Complete blood count & blood culture done - discontinue using amoxicillin. Hope I have answered your query. Let me know if I can assist you further. Take care. Regards, Dr. Yogapriya V, General and family physician.