I've had absolutely poker-straight hair my whole life, but recently I've found a few hairs taking on a curly texture. I've eliminated all the obvious things-- change in haircare, kinks from styling, etc. Some of the strands feel really dry and damaged, and these ones tend to have a rougher texture and a less defined curl, but most of the others are as well-nourished as the rest of my hair. My hairdresser says my hair is in perfect condition.
Changes in hair texture have more to with genetics than the hair care products you're using. It's probably related to hormones or just plain old aging.
Maybe some kind of hormonal medication, like for birth control, could be causing it. Or a change in the brand of birth control. Whatever it is, I hope you don't mind getting some curly hair! I personally think straight and curly hair are equally pretty.
Going through puberty made our daughter's straight hair go (beautifully) curly. But considering you are 19, I doubt that is the cause of your change from straight to curly. Have you mentioned this to your doctor? If you haven't, perhaps you should at your next appointment.
I mentioned it to my hairdresser but since I very rarely have to see a doctor I haven't discussed it with them yet. It's not really a concern for me health-wise, it's just strange, so I'm curious as to the cause. I would have thought that genes would be genes, and hair texture would be pretty set in stone, but the body can do weird things, I guess!
Curious if you are taking on grey strands with the new texture. Have you asked your parents if their hair texture changed when they were about your age? This might be where genes come into play.
I started out with straight hair. By the time I was a teenager I had wavy hair (puberty I'd guess). Then when I had my son at 29 my hair became curly. Now that I am 49 and in menopause, my hair has become straight again. So my hair texture has changed over time whenever my hormones changed. Hope you solve the mystery to your texture changes.