Am I eating enough/overtraining??

Hello everyone:
(5'3)
My more normal weight is around 103-105 pounds, but I currently weigh 92 pounds. I lost the weight in a span of five months recently due to exercising more than my usual. I find it hard to believe that I lost so much weight by just walking at a fast because I know some people run for miles and don't get sore like I do. I also ate more but it seemed to cancel out. I did it compulsively due to stress, but I want to find a balance because before I didn't workout as much. I do feel sore a lot of the times, but don't understand why maybe everybody is different and it's too much for me even though it's not super rigorous. Here's an overview of my situation.

My activity level before:
98-105lbs (1,600 calories)
-Walking around house, lots of walking around university campus, taking the stairs, rarely went out for walks on the weekends
-My 15min cardio and strength/yoga routine

What made me lose crazy weight
92lbs (eating 2,100 calories)
-All of the above plus walking at 4mph for an hour daily and a twenty minute walking session

Now
92lbs (eating 2,100 calories) WANT TO GAIN WEIGHT
-Like my activity level before but still keeping an additional 20min walk