Not everything is easy to detect on a mammogram and even on an ultrasound especially if you have dense breast tissue. I would suggest definitely pushing for a breast MRI ; even if it costs you some money it may well be worth your peace of mind . Be a squeaky wheel until you're happy ! If it's so sore you can't put your arm down and getting worse over the weekend you could always go to casualty and they may be able to push things along for you . All the very best, hang in there and I hope you're all sorted very soon :) Karen