Its worth taking the floor out once a year as sand will collect under it. I even find the odd escaped mackerel under mine..and that improves the smell of the SIB tremendously. The bulge may even be water collected and pressing on the floor beside your inflatable keel.
The keel is just a long thin tube that runs down the centre of the boat under the floor..and its not attached all the way down.
To get the floor out..follow the below vid in reverese.
Deflate your tubes..you will see a couple of aluminium “runners” at the edge of the floor “panels”. They slot over the edges of the panels to hold them rigid. Possibly you may need to loosen them by putting a wooden block on the runner then smacking the wood with a hammer. I sometimes have to do that to break the seal if its not been out for a while. Once the runners are out from both sides of the floor panels..lift the panels. Be prepared for a few skinned knuckles the first time..but its easier with practice.
Perhaps try tipping the SIB up first..to see if it is water under the floor that is causing the bulge..drain it off if that is the problem. Or try deflating the keel and slapping the bulge to try and flip the keep back into position..then inflate again. That is if you cant get the floor out..but best if you can.
Hope that helps some
Although this vid is not a quicksilver..they are all much the same. You can see the keel near the start.