Only 16..

My boyfriend and I have been together for a year and six months and began having sex on our one year anniversary. We have yet to use any sort of birth control the entire six months. I thought I was pregnant last month, but took about 4 pregnancy tests and they all came back negative. I also had what I thought was implantation bleeding during the early weeks. I didn't have my period that entire month either. I've always weighed around 114-116 pounds, but recently I've been gaining weight like crazy and all I do is eat. I now weigh 128 and I have began to see stretch marks around my nipples.. I'm just very stressed. We have sex during my fertile window all the time without realizing it and I'm just scared I'm pregnant or will soon become pregnant. I have a job, but since I'm under 18 I'm only making $7.75 an hour. That is NOT enough to raise a baby... I've already consulted my mother. She took me to the doctor at what would have been 3 weeks, the test came back negative and since then she's kind of just dismissed the idea.
I'm in desperate need of advice! Preferably a few success stories from mothers who were young when they had their first baby.