Hey velma, sorry for late repost.
You know that for some women it takes years to get a regular period? You are only 18, so your periods could still be irregular, in my opinion if you were 28 you would have much more regular period cycles. Maybe you took some pills? Try to remember, or if you are on some birth control and you didn't mention it then we probably found an answer to your question. Did you have some stress lately or any other factors that could affect your period like new hobby, sport etc. Try to give more information. Maybe other symptoms like weight gain or loss? Anything. You should know that there are many factors that can make you have irregular period or even its absence. I'm truly shocked that your doctor couldn't provide you with some useful information, at least he or she should make you an appointment to a gynecologist if you already visited one, maybe you should try to go to a different gynecologist? You would have two professional opinions to compare. Try to fix this up because 4 month without a period is a long term, even 5 now, or it came already?
Hope you'll be O.K. Best wishes and be healthy