Difficulty maintaining weight on new program

I have recently begun a new program and am having a hard time adapting to my new physique. A little background; I have a 3 year old and an 18 month old and have been pregnant or nursing since 2012. I weaned my youngest down to one nursing session/day back in July, and my weight then fluctuated between 100 & 103 (at 5'3.) After my 3 year old started school in September, I decided it was time to get into a workout regimen in order maintain my weight that had evened out at 105, build back the muscle that had been lost in recent years, and get myself into a healthy post-baby routine. I am on a low dose of Levothyroxine for Hashimoto Hypothyroidism and another low dose of Zoloft for anxiety, but am otherwise healthy. My problem started when I began my new workout routine..I do a variety of workouts, 3-4 days a week. These involve yoga, Barre, fusion and aerobics, using mostly my own body weight resistance. I also began monitoring my food intake, (even cutting down on my beloved Rocky Road) although I generally maintain a pretty healthy diet and have always taken in more water than needed. In the weeks since, my weight has jumped a bit, and I now fluctuate between 107 & 110. This was to be expected, as I gained muscle, the real problem, however, lies in the fact that I've also gained a couple of inches all around. Enough so that I'm teetering on a new pant size after 10 years of wearing the same. I can handle the weight, but why am I so much bulkier?! I've always had curvier hips and thighs, but now my pants are crying out in agony!! What's the deal?!

I await any advice with bated breath (mostly because I'm sucking it in