hey.. if you had unprotected sex, get a blood test for pregnancy concern, if not then still get normal blood test..but do see a doctor.
take care :)
I would go see your doctor as there is something going on. If you are on the birth control pill, sometimes it stops periods and you need to be on a different one. Sometimes home pregnancy tests are not sensitive enough to show positive. If your breasts are sore and tender, there is a chance you are pregnant even with a negative home test result. In all cases, you need to see a doctor because at 18 your periods should not stop like that. I would go back to the doctor that prescribed the birth control pill if you are on one, or your regular doctor or to a clinic to talk to a doctor. Your doctor may refer you to a gynecologist, which is a specialist in women's hormones and reproductive organs.
Take care and I hope everything is ok