Weight gain and muscle tone

okay, i just had a baby almost 3 months ago. i know breastfeeding burns calories and makes you lose weight, but i have lost TOO much weight. i'm 5'3, 107 lbs, and my bmi is 19.5. even though i am considered in a healthy range, it's too skinny for me and i weighed more before i was pregnant. i have increase my calories with peanut butter, more bread, and a half-serving of a weight gainer once a day. now, i don't wanna be fat, but i'm afraid to exercise because i don't wanna lose weight. can i do strength training/toning exercises instead? or should i still do cardio? also, i want a tooty booty, so what are so great exercises besides squats that are good for a nice butt?