This is an ambiguous question. It could be legitimately interpreted either way. That is to say: both meanings are correct. I suppose if it were spoken, the difference would lie in which words were emphasised. If you emphasise the "I", it's more likely (though not guaranteed) to mean the former, if you emphasise the "Why", it's more likely to mean the latter though, again, it's not guaranteed. Emphasising the "why" could equally mean something to the effect of "I can't understand what your reason is for thinking I did this".
Anyway, to answer your question: it means both. It depends.