As long as your weight is within the normal range for your height and age, you're fine! The range is quite wide.
Here's an online body mass index calculator. But, do enter your birth gender, since birth men have different dietary needs than birth women. Both underweight men and women risk developing anemia, osteoporosis, immune system deficiencies, and infertility problems. Oral health is affected too.
http://www.halls.md/body-mass-index/av.htm
