Hormonal shifts occur many times throughout life including during puberty, pregnancy and menopause. These changes can cause many changes to occur in the body, including the shape and thickness of the hair follicle. In other words, while your hair once was smooth and sleek, a hormonal shift can cause it to become curly or for curly hair to become straight.
An important thing to remember is that you should always use hair care products that are suited to your current hair condition. So if your curly hair has become straight, you should swap your shampoo and conditioner for one that is specifically for smooth and straight hair.