Advice

By nature Im a thin person. I was 124 before getting pregnant and then with both of my pregnancies I gained just 17lbs and had average 7 pound and some oz babies. Im breastfeeding and keeping up with a toddler so my calories are burning like crazy! I eat as often and as much as I can but Im still at 121.4 and Id like to gain about 5-6 pounds and keep that on. How can I do that? What foods would help? My metabolism is nuts and even before babies I sat at 125 for just about 10 years. Just dont want to look too thin.