Within the last 1.5 yrs, I lost 50 lbs - the last 10 was a few months ago, but it was unintentional (I started a new med that zapped my appetite). I'm still a healthy weight for my height, and I'm not concerned about it, since my weight has leveled out since then. However, I lost weight in my upper body moreso than my lower, and so my chest looks very thin while the rest of me looks normal or flabby (butt and thighs

) I started getting comments about how I look too thin (I'm 5'8" and 130lbs) - I think it's a combo of people (mostly family) being used to seeing my 50lbs heavier, and this "new" look of my chest/upper body. I've actually become quite self-conscious of it b/c you can almost see my rib cage, especially across my upper chest, but also in the back/sides somewhat. I lost a
lot of breast tissue when I lost weight (34DD to 32C), and since I'm not muscular, all that's left is weak muscles and skin
So, the reason I'm here is for suggestions on how to build up some muscle mass on my chest and shoulders. I really want to tone up my legs and butt, and so I'm trying to do running and some strength stuff, but I don't know how to best attack my chest. I have weak shoulders (bursitis), so push-ups are hard (though I'm trying to do some modified). I've always wanted to focus on my lower half, since that's always been an issue and self-conscious area for me, but I never thought my chest would become something I'm worried about! And I just don't know how to approach it!
[I should note that the majority of my weight loss came from diet changes, not exercise, and that's a problem for me now because I want my body to look as good outside of my clothes as it does inside of them. I'm in school and it's very time consuming, and I know that's not a good excuse, but it's been hard to do - I'm looking forward to warmer weather b/c I love running outside and I find the nice weather to be a huge motivator]
Any suggestions would be much appreciated!
Thanks!!