My mom had silk straight hair until she had children then her hair started going curly.

Get your hormones checked. Are you doing anything to damage your hair? Dying it, using heat, etc. that could be messing with your natural hair texture & causing frizziness which can be viewed as a wave or curl.

What hair products are you using or have you changed them? Check the ingredients in your shampoo, conditioner, leave in, styling products, etc. if they previously contained silicone and your new products don’t you could have had wavy hair all along, but the silicone was weighing your hair down.

If you’re just air drying your hair most people with straight hair to have bends and waves to it. Are you going to bed with your hair wet? Or leaving it in braids/buns?