Anyone ever watched the bottom of the hull flex while applying backstay? Is this ok?

Here's what me and JP noticed while fixing our backstay system. Our backstay, like most of the boats I think, has 2 main attachments: 1 is the deck hull joint just port of the tiller; the 2nd is on the floor of the boat, where there are 2 little fiberglass bulkheads just to starboard of the exiting scupper where the floor meets the transom. There are little chainplates on each of the little bulkheads. The starboard line of the backstay divides and one end attaches to one chainplate and the other end goes through a block attached to the other chainplate and starts the fine tuning system. After we fixed everything, one of us was below looking at the attachments on the little bulkheads, while the other one hauled on the backstay from up above. When the backstay is on, the floor of the boat lifts!! And when you pull it on and off, the floor just goes up and down. Yikes! Does this happen on anyone else's boat? Is this ok? Thanks!