People vary enormously in their range of frequency, twice a day to twice a week is well within normal expectations at your age.
Not everyone gets morning wood, however, everyone usually has one or several erections while sleeping. Try the postage stamp test for a few nights, if you fail that for two nights in a row, you've slight cause for concern at your age imo.
However, you're 99 lb and 5'7" (BMI 15.5, far below the healthy range of 18.5-24.9)
THIS, is something you should consult a doctor about (your weight being at an unhealthy level). Do you smoke? Even so, your BMI's still far below the healthy range.
Thanks for reply.
I don't smoke.
I know my weight is below normal. So... Is there something I should worry about (except weight)?
Your weight being unhealthy is something to worry about.
And one of the many symptoms and harmful syndromes likely to result from being so very underweight might be its impact on your frequency.