An absence of the normal menstrual period, called amenorrhea, can be caused by many different underlying causes. In order to properly review your case, it will be important for you to schedule an appointment to see your doctor
so that he or she can obtain a more complete history, perform a physical examination, and run any tests that may be appropriate.
Having said that, the first test that is usually ordered when someone is experiencing amenorrhea is in fact a pregnancy test. It is often difficult to determine whether you are pregnant by physical changes and sensations alone, such as morning sickness, breast tenderness, or absence of period (as mentioned above). That is why a pregnancy test is often recommended. Your sexual history is a very important factor in determining your risk of being pregnant, and includes when your last sexual encounter took place, what type of contraception
you are using (if any), if you use contraception every time you have sex, and how long it has been since your last menstrual period.
You are also correct that being underweight can affect your menstrual period. Oftentimes athletes who train rigorously can go without having a period for many months, and it is the same for those who are underweight for other reasons. This triggers the hypothalamus to release decreased amounts of certain hormones that then decreases the amount of luteinizing hormone released from the pituitary gland, which in turn decreases the amount of estrogen released from the ovaries, leading to an absence of your menstrual period. If you have been exercising more often than normal lately, or if your weight has been decreasing, these are risk factors for this condition.
Again, it will be very important for you to see your doctor so that he or she can perform a complete evaluation to determine why you are not having your menstrual period. Signs and symptoms alone will not be able to tell you what the underlying condition is that is causing your amenorrhea.