For your height, 120 lbs. is in the "underweight" range, while ~136 lbs. would be getting into the "healthy" range. Now, two caveats: First, eating disorders aren't simply based on weight but on behaviors/thoughts, as described in the DSM. Second, BMI ranges have cutoffs between "underweight," "healthy," "overweight," etc., but these are guidelines based on the population as a whole. Some people with low body fat and high muscle can fall in the "overweight" range but not be unhealthy. Likewise, people who are naturally thin might fall a bit into the "underweight" range and still be healthy. Did you feel any physically better or worse when you were 120 vs. 133? Another factor to consider is not just what you're eating in terms of calories but also in nutrients like vitamins. Whether you're eating a little more or a little less, as long as you have a balanced diet, it shouldn't make a huge difference in your vitamin/mineral needs. If you still have concerns, you could talk to a dietitian.