All of these are good questions to be asking, but there is much more that goes into becoming pregnant than just your size. For this reason, if you are thinking about becoming pregnant it would be good to meet with an OB/GYN or your primary care doctor
to see what you can do to best prepare both yourself and your body. Obviously, having a baby is a big deal that will lead to many changes. By making sure that you are in your ideal state you will have the best chance at a successful pregnancy and a healthy baby. Dietary changes, nutrition supplements, avoidance of drugs and other social habits can all be an important part of the process before seeking to become pregnant, and getting the correct council early on can be vital.
With specific focus on your question, you are correct that most of the literature on obesity and pregnancy focuses on people that are more obese than yourself. That being said, people that are too thin can also have problems, and so it is important to be at your ideal weight. Certainly the fluctuations in your weight could play some role as well, but the key is that you are still very early in trying to become pregnant. Please speak to your doctor for appropriate guidance.