Advice?

I have recently lost 20 pounds all of a sudden.i mean like in a few months time. I wasn't eating healthy or exercising so I don't know how. I asked some people and they said it could be stress or me just losing baby fat. I had just turned 18 and moved out. Every since I was 12 I have been 5'4'' and 140 pounds. Now I'm still 5'4" but I'm 120.