Health issue putting a damper on my workouts, what to do?

So, I'm a pretty thin person but over the wintertime, I do get a little soft around my thighs, tummy, (in place of my normal toned). I do lighter workouts in winter and increase them during the Spring to tone my body before Summer. 

However, recently my chest has been hurting and I went to the doctor and they told me I have costochondritis (inflammation of the ribs) and I should only stick to yoga and stay away from strenuous exercise. 

My issue is how am I supposed to get toned now? Are there any light workout regimens that won't affect my ribs that I can do? I don't even know how to do Yoga, can it tone my belly? Again I don't have much fat whatsoever, I'm 5'1 ~100 lbs woman but I eat like a freight train (call me Shaggy from Scooby Doo) and I don't want to be soft and squishy for summer if you get my gist? My weight actually usually increases after a few months of strenuous exercise. 

Any help is appreciated.