You do not owe anyone your full medical history. You do get to decide what you share, with whom, and when.
A simple opening script:
"I have a medical condition called sickle cell disease. It can cause pain and fatigue that sometimes affects my energy. I am sharing this so we can work together if I need to adjust my schedule on harder days. I am not looking for special treatment - I am looking for understanding."
Remember: flexibility is a right in many jurisdictions (reasonable accommodations, protected leave). It can help to talk to an advocate or occupational health professional before a formal conversation.