118 at 5'2" is not that big so my guess is that you are putting on muscle. Your glutes will definitely round out and your thighs will change shape. If you're noticing that your pants are tighter closer to the knees, that's very likely new muscle. Most people will find that they lose fat off the upper part of the thigh and gain muscle in the lower part.
Good job on getting out there and doing it!!
PS - even with back pain you can get a road bike, it's just a matter of finding someone to fit you properly. You can get special stems and headsets that bring your body position more upright, even on a road bike. Then you could still take advantage of the lighter faster bike with the comforts of a townie.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS