These emotions are due to hormonal changes which occur during this time in the body. If you are having regular menstrual cycles, then nothing is to be done. Psychological counseling is what is desired at this stage.
Other than that, if your periods are irregular then we can start with the hormonal contraceptives.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I remember the same thing at your age. I would cry and cry at night simply because I was lonely. If you are still sexually pure and have not messed around, KEEP IT UP! The body does push you to do things, but you need to control those urges. They do lessen in time, I assure you. I'm 22 now, and, having been working with children since I was your age, it has helped me to feel fulfilled. Also, you need to have a personal relationship with the Lord Jesus Christ. Do you have a Bible? Read it. Read about how much the Lord loves you, what it means to live a Christian life, and how single Christian women have served the Lord in the church. Commit yourself to Him and His service and you will feel fulfilled, though your life won't always be easy.
It sounds like hormonal changes through your cycle. I have wanted a child since I was 12, though obviously I acknowledged it was a bad idea. Even at present, it's not a good idea for my boyfriend and I to have a child as we're not financially stable on our own to care for the child and I want to finish school first. Oh, I have the urge, believe me, but I won't give into having one. He has a 2-year-old niece and a nephew on the way, so I'm able to spend time around children to help at least a bit.
So, I agree with Jessika that you should definitely control your urges to have a child, but I believe it is your choice whether you decide to have sex should you get a boyfriend. It's also your choice not to have sex if you get a boyfriend, so don't let a guy or your urge for a baby to pressure you if you don't believe it's right. Sex itself is not going to fill that gap or fix those emotions. It's only going to provide a false sense of closeness unless introduced when you are both committed to each other.
I would suggest looking deeper into the issue either through therapy or a journal to figure out what this urge really is, though, so you can figure out another way to fulfill that "missing gap." When it first started out, my urge to have a baby truly was an urge to be loved/have someone to love unconditionally because of familial/life circumstances. It wasn't until I found a fulfilling, loving relationship with my boyfriend that the urge really dissipated. It's only really been the past few months the urge has really come back, and it's largely due to the fact we both have admitted to wanting to marry and have a family when we are financially able.