Previous Benign Microcalcifications/ Now Solid Mass

At age 33 (7/2010), I had 2 separate surgeries to remove microcalcifications in my left breast. The first showed nothing, the second they reported them as benign, YAY! I had 3 and 6 mth follow up mammograms thereafter, with no changes. At the age of 38, I felt a small lump and reported it to my gynecologist. It is on the left side in between my armpit and breast. The doctor scheduled a mammogram for me and I was told they only saw tissue where the lump was. An ultrasound was even performed.....just tissue. I was then told that I would not need to come back for another mammogram until I was 40. I was happy with those words and moved on. On 07/7/2016, I was at the gynecologist and mentioned the lump again, and let her know I could feel it more now, and wasn't sure if it was because I was getting older or not. She scheduled me another mammogram on 07/20. This time the mammogram actually showed something. I then go in for an ultrasound same day. The tech left the room and came back with the doctor. She told me they found a solid mass that was not on my previous mammograms, and they wanted to do a needle core biopsy. She also offered me the option of having it removed since I had had two surgeries on the same breast as before. I opted for the needle core biopsy. They scheduled it 2 days later. Didn't feel a thing except when they numbed me up and that was nothing. Had a conversation with the dr while he was performing the biopsy, which helped ease the anxiety. Had a mammogram to ensure the marker was in place and was on my way. Now waiting on results.... Im just a little worried because they just seemed way to calm, but yet urgent to get all of this done. One good thing is they mentioned I would have my results by late today or Tuesday. I have a question though.... Is it possible that they missed the lump the first time because they did not have enough of my tissue in the mammogram the first time? The lump is positioned in a spot that made it hard for them to get it in the machine, and its up against my ribs. They have to almost get my armpit in the machine to get the picture.