“My cylindrical number is changing in nine months. What could be the cause for this?”

I am 16 years old and I have had myopia for the last 8 years. My spherical power of -2.5 and -2.75 is stable for the last 2 years. I got astigmatism 2 years back with -0.25 power in both eyes. Since then my cylindrical power is increasing -0.25 every 8-9 months. Many of its symptoms such as night vision problems, blurred vision, and sensitivity to bright light are present for at least a year. I have no history of it in my family. Could this be a serious issue? Could I have keratoconus? Should I consult an ophthalmologist specializing in the cornea? What causes astigmatism in the first place aside from genetics?