Injection for sentinel node biopsy

Hi there got breast cancer for the second time in 5 years. First time DCIS right breast and had lumpectomy and radiotherapy. This time lobular cancer left breast, quite large and booked for mastectomy 12 June. Not so worried about the op but getting stressed about having the contrast dye injection which is arranged before my op. Had it last time and no problem as breast was numb from the lumpectomy a few weeks before. Have heard it is really painful. Has anyone any advice to give me? Just want this all over and done with and be able to start recovering x