Mystery solved!

I feel so silly for forgetting. I planted a hill of Heirloom Winter Squash. That must be what this is! It just keeps getting bigger and rounder every day. It has looped itself in through the trellis fence, so there's no way for me to put it closer to the ground to keep it safe from gravity. I've put a sling around it for now, but may have to improvise as it gets bigger. I'm so glad I solved the mystery!