If it's been 28 days since your last period, and that's your usual cycle, it's not too early to take a pregnancy test. (Generally, you can take a test and get a reliable result as soon as you've missed your period.) If you take one of the OTC tests and you don't trust the answer, you can go to your doctor and have them administer a blood test, which will give you a definitive answer. (If you don't want to go to your regular doctor, I highly recommend Planned Parenthood, which can give you the test and then counsel you on your options, to include parenting or adoption - PP isn't just about abortion.) Mom moment here: please take this as a warning bell, and while you're at the doctor, talk to them about protection. You should be able to get a presecription for most contraceptives at age 18 without a parent's permission. If you plan on having sex again and you do NOT want a baby, please do this as soon as possible.