When you're head over heels, you're confused or thrown off by something. People say they're head over heels in love when they feel disoriented and swept up by their romantic feelings.
You can use head over heels for a particular state of disarray, although it usually shows up in the context of love, and it's come to mean something closer to "extremely." Your parents are lucky to be head over heels in love after being married more than twenty years. The original phrase was "heels over head," which makes sense, since our heads are normally over our heels. For some reason, it was reversed to head over heels in the eighteenth century.