How Should A Breast Lump With An Enlarged Lymph Gland Be Treated?

I have breast lump in the upper outer quadrant of my right breast. Approximately in the 11 o'clock position. The mammogram and ultrasound did not detect it but detected an enlarged lymph gland (8mm) in my arm pit. The doctor said every thing was alright but I feel a proper breast doctor should be consulted. Is it possible for the lump to not show up in both procedures? What do you think I should do? Thankyou for any feedback. By the way I am 56 and 4 months from being menopausal.