It means you are falling over, or tumbling forward, like a gymnast, because you are so in love.

Ian is right about the meaning, but I agree with you that it's strange to describe someone who is toppling over as "head over heels," since, as you say, the normal way to stand is with your head over your heels! In fact, the phrase was originally "heels over head," which makes a lot more sense. But for some reason, the phrase changed to "head over heels" a few hundred years ago (in the 1700s), and that's the way everyone always says it now.