This is a warning sign of a serious structural problem that will need to be taken care of immediately before the damage gets worse. The surface you walk on should be level, safe, and secure. Any dips or noticeable sinking are issues that point to a problem with the crawl space down below.
Whenever you don't have the area beneath your home properly encapsulated (sealed) off from the outside environment, it can let in a lot of moisture and excessive humidity. When this happens, it's only a matter of time before the moisture in the soil and in the air start to cause damage to wooden materials, including support beams, posts, and floor joists. Damage here is what will cause you to notice that something isn't quite right when walking across the floor.
Crawl Space Stabilizer
St. Louis Basement Pros can install The PowerPost from Nash Distribution which will provide the strength and stability needed to raise the first floor of your home back to the level where it needs to be. Our techs will be able to determine how many posts are needed, and where they should be installed based on where the damage is located with the wooden materials.