I'm guessing you didn't take the last advice given to make an appointment with your doctor. Your health is not something you want to mess around with and you need to make sure you are fully healthy and if you can possibly be pregnant, it is even more urgent that you need to see your doctor. If you continue down this road, you will end up in a bad place and can have multiple disorders if you don't get it together. My good friend was underweight all through high school going into college and never did anything about it. She now lives with a number of digestive disorders(leaky gut syndrome, crohn's disease, IBS, etc). It's not fun and she's on daily medication. Make sure to get yourself healthy by going to see your doctor so you won't follow the same fate. Take care.
I'm also underweight and I have two healthy children
Why haven't you tested? You've been thinking you're pregnant for over 6 months. This IS NOT something you just put off and ignore just because you're scared. IF you are, baby needs care and it needs it now. Lack of prenatal care can cause numerous problems in the child, and cause them to be stillborn. Take a test!!! Stop guessing, and stop putting it off. YES you can get pregnant with a BMI of 18. Mine was 17 and I got pregnant, however I miscarried. Think of women in 3rd world countries. They get pregnant being super underweight and don't have good food OR clean water.