It feels like an accumulation of fat( a little pillow) under arm. it's been 5 weeks since mastectomy. will this leave? expecting to much too soon?

Hard to predict: Swelling under the arm or all over the arm does occur to a variable degree after mastectomy, especially if the surgeon removed all your axillary(Arm pit) lymph nodes. Have you shown this to your surgeon? I think it is Lymphedema...verify it and ask your surgeon to help you in managing it. I hope you are exercising your swollen arm daily. This may help. Seek help from a Physical therapist.