Just saw this thread and when I saw your pictures it took me back! Your babys arm looks just like my little girls did almost a year ago. I would say it is a hemangioma as well. At birth, our daughter had a faint, flat, sort of discolored area on her arm and within a few days it started turning more pink and then progressing to red. Long story short, it took 2 1/2 months to get an appointment with a pediatric dermatologist and her hemangioma grew rapidly within that time. We started Propranolol at 10 weeks and we could see it working within the first week. I feel like the earlier you can start some form of treatment the better. Had we gotten on the medicine quicker I feel like our daughter would have been saved from a lot of pain and complications from her ulcerations.
Keep us updated! Let me know if you have any other questions or would want to see any other pictures to compare to since their birthmarks are in the same area and so similar. Good luck!