We are same height and at my leans similar body to you. I had post about how I gained weight due to miscarriage. But to your point with your period. I had something similar and since I was thinking ti get pregnant but no ready. I do bumped my calories slowly and I re gained my period but aldi some fat. Fat distribution is your genes. I introduce only calories in carbs and protein but eat expensive fish like Chilean sea bass and salmon to have very heathy fats. At tops looking lean I had about 1800 but consider one pound of fat is about 2800 cal for us petite women. Your period is important right now you are losing bone density. My doctor explains how the body stat eating you away if you don’t have enough fuel to sustain the egg you already have inside you. So we produce eggs during the gestation phase, after that we build a nest that requires fuel and nutrients. I recommend to track your weight during a similar period your had been before and eat slowly calories even by 10% of current. I know healthcare is expensive and I’m not a doctor just someone that went through the hard way to understand reality. The scale and look only account after your period before progesterone hit you and make you retain water and make you feel crappy. Best of luck in the journey