When someone falls they almost always are embarrassed and say I'm OK. That's just a normal reaction but one that a department store will ultimately try to use against you if you make a claim. It's important to try and keep your wits about you. There are some big box stores that actually discourage accident reporting and sometimes even refuse emergency care.
If you fall and even think you may be injured report it. Insist on emergency care if ne.eded. Don't immediately assume it's your fault. Take the time to assess the situation--was the floor wet, did an object trip you? Take photos. Don't assume the store will do any investigation or document what caused the fall. Get witness contact information. I have had cases in which the one of the largest stores in the country deliberately ignored independent witnesses and failed to follow their own internal procedures for investigating and documenting accidents.
While it's best to seek immediate care you may not realize you're injured until later. If so get treated as soon as you recognize you've been hurt.
Many business owners insurance company's adjusters call victims and ask for a statement. You should politely decline and call an injury attorney who has experience with these type cases.