A Priori Self-Knowledge: A Real Pain...In the Head?

Here is a question that has recently been giving me a headache, and I'd really appreciate getting your feedback on it. One major criticism of content externalism is that it undermines a priori self-knowledge. When such critics say that self-knowledge is a priori I take them to mean that self-knowledge is independent of experience (i.e., not based on empirical observation). However, it is not clear to me that self-knowledge is essentially a priori. (This is a point that I believe has been made by Crispin Wright.) Suppose, for example, that I were suffering from a migraine. Presumably, my knowledge that I am currently suffering from a migraine is a type of self-knowledge. However, is my knowledge that I am currently suffering a migraine independent of experience? The answer seems to be ‘no’. I can only know that I am currently suffering from a migraine if I am currently experiencing the migraine. Thus, my knowledge that I am suffering from a migraine is a posteriori.

Am I missing something here? Could there be an alternative definition of a priori according to which my knowledge that I am suffering a migraine counts as a priori? Or am I mistaken in claiming that my knowledge that I am currently suffering from a migraine is a type of self-knowledge? Or perhaps I am missing the point of the critics of content externalism altogether? I should add that the issue of a priori self-knowledge is separate from the issue of infallible self-knowledge and authoritative self-knowledge, so that even if I am right, this does not mean that the content externalist is out of the woods just yet. But presently, I am only concerned with whether a priori self-knowledge is a type of self-knowledge with which the content externalist needs to be concerned.