Weight loss after stopping birth control

So my fiancé and I agreed about 4 months ago it was time for me to stop birth control. We are both in our mid 30's and have no children. I had been on the pill since I was 18 and needless to say very over it. We are not actively trying to conceive but leaving as an option at this point. However since stopping the pill I have lost about 15lbs. Now I know normally this wouldn't be an issue except I have always had issues with being under weight. So 15lbs is a huge number for me. I now weigh 100lbs and have tried increasing my calories, carbs, and protein intake but still losing weight fairly fast. I know healthy weight is one of the things to maintain while trying to conceive but seems impossible for me. So anyone who has been in the same situation or something like that I would really appreciate some feedback because I'm out of ideas and would like to know I'm not the only one. Tyia