Hello and welcome to HealingWell. I am sorry to read of your pain as well as your dx. Do you generally have pain with your CML and could the pain be from your sternum ( bone pain).
Muscle strain could cause your pain and you have stated that you have been working out really hard for you. Even with light weights, if your body is weak from chemo and you already have had heart problems, it may not take much of a work out to cause a strain.
Your history and your symptoms are a challenge when trying to sort out what may be going on. I know chemo can cause just about anything as I am on chemo also. However, your heart would be the place for your Drs. to start when trying to identify what is causing your pain.
Do you happen to keep a pain journal so that you can note what you are doing, time of day, intensity of your pain, what makes it better and what makes it worse?
My best advice, be your own advocate and if you feel you need to be seen re this chest pain, present to an ER, call your Cardiologist and make sure every thing is being done to try and help you with the crushing chest pain.
Thinking of you and hoping you are soon rid of the pain.