When someone slips or trips and falls they usually are embarrassed and say "I'm OK". That's just human nature. After the dust settles you may realize that you're hurt. It may even be later that day or the next.
Even if you are not sure you've been injured the accident should be reported. That may allow the store or premises owner to correct the problem and save someone else from getting hurt. You should also take pictures of whatever caused the fall and get contact information from any witnesses. I have had cases in which store employees ignored independent witnesses and failed to perform any investigation despite internal procedures to the contrary.
Unfortunately whatever you said after the accident can and will be used against you by that "big box" store if you make a claim. Store, restaurants and business hate to see anyone make an injury claim. In fact there are often financial penalties to employees and managers for injury claims.
If you have any question about being injured let the owner know. Should you know you are hurt request emergency care, insist if need be. For those who are stiff or sore later, go to the ER or your family doctor.
Anyone who suffers an injury after a slip, trip and fall should call an experienced injury attorney for a free consultation. If an insurance claims adjuster calls for a statement I recommend that you politely decline and get legal advice.