120lbs is a healthy weight... Especially if you are breastfeeding, you still need extra calories to keep your energy levels and milk production up.
Also hun, you will find once your body continues maturing you will weigh more. I am by no means fat, wasn't before pregnancy, but my body and weight at 16 was definately alot less then once I hit 18, 19, 20... 15/16 I was steady about 115lbs and I am 5'3. 20 years and up I was steady 125lbs and am still only 5'3.
Your metabolism slows as you finish puberty etc (which doesn't end until your 20's). Your body has likely matured alot since last year.