Well, you're on neither of the things I mentioned, then, but I think you'll still ineligible to visit the US without a visa if you overstay. The thing is, you don't know how complete their records are (did you cross the border over land?). They probably scanned your passport when you last entered either way, so the absence of a stamp does not mean they don't know you're here. When you go back, they might not scan your passport. If you walk across the border then, they will never know when/if you left, and might have to take your word for it the next time you try to re-enter (based on your "good" past behaviour). Still, I would avoid overstaying. Can you exit and re-enter? Mexico? The Caribbean? Somewhere.
Also, you do know you were given leave to enter for 6 months, right? Not 90 days.