If you are worried about it (and it sounds like you are), it is fine to go ahead and have a biopsy. It doesn't have to show up on US, there are mammogram tables that allow them to do what is called a stereotatic biospy - basically they do a mammogram, use a computer to localize the area, then use the computer to guide the needle to that location and biopsy it. Some women have masses that they can feel, but aren't seen on mammogram or US and they are biopsied based on palpating the area.

If the radiologist and techs aren't worried about it, it will probably be fine to wait six months. I would say do your monthly breast exams, and if you feel any thing growing, or feel anything in your axilla (a lump of lymph node), go back in and have them biopsy it.

What ever you decide to do, I hope you can have some peace of mind about it. Chances are it's nothing, but it is something to keep an eye on.

Good luck!