Is it possible for a Certified Medical Assistant to work as a phlebotomist in a hospital?

Yes you could, however you should have received a phlebotomy certificate with your MA stuff-they may want that. However, I would be cautious about leaving your current job to do this. A phlebotomist usually does not do much other than draw blood, and you will be out of the lab, not nursing. You will not have time to hang out with nurses. You will run into the patients room , get the blood and go back to the lab. It really is not interesting. It is a toss up if you will make more money than the $14/hr you are making now. IMO, you are in a more interesting and better position now.