Am I undereating?

I'm 5'5" and probably weigh around 120 or 125 lbs (I stopped weighing myself when I realized I was heading down a borderline eating disorder path). I exercise on an elliptical for around 60 or 70 minutes 6 days a week and do some light strength training a couple days a week too. I'm also usually pretty active; I spend a lot of time on my feet, and now that it's summer, I've been spending a lot of time swimming & hiking. I don't have a whole lot of body fat, but I do have a lot of muscle mass. I don't know how accurate it is, but MFP tells me that my BMR is around 1700 calories and my maintenance a little over 2000 (including my elliptical workouts but not the swimming or other outdoor activities).
I was eating less than 1700 for a long time. I lost my period, felt tired all the time, and had no energy. So a few months ago I started increasing my calories. I think I gained a little at first but then everything evened out again. I've gotten my period two consecutive months in a row now, which hasn't happened in over a year. I noticed I had more energy, was in a better mood, and my legs didn't feel nearly as fatigued as before. I probably eat right around 2000 calories/day now. But for the past week or so, I've been feeling the same way as I did before increasing my calories. I feel drained, lightheaded on occasion, my legs feel heavy while I'm on the elliptical again, and I'm in a bad mood. Usually by the end of the day (like after dinner), I start to feel better/have more energy but sometimes even after eating dinner, I'm still feeling not so great. Should I be eating more? Could I really need over 2000 calories to maintain my weight?
Sorry for the long post. Any advice would be appreciated!!