found lump in breast, ultrasound showed nothing, but lump is still there and bigger

I am a 26 year old woman and found a lump in my breast over a year ago while pregnant with my second child.  The doctor dissmised it at the time saying it could just be milk ducts clogged from being pregnant.  I since have given birth and breast fed my child for a year.  Since the lump could still be felt, I had an ultrasound done while still breastfeeding my baby.  Nothing was found (after asking my age, the ultrasound technician did the procedure in less than 2 minutes and only checked a very small section of my breast, which seemed lower than were the lump is actually felt...  my breast were also full of milk that day so hard to feel).

I have since stopped breastfeeding and the lump is now bigger then ever and very hard to the touch.  Should I see my obgyn again?  Or should I go see someone else?  If so, what type of doctor?  Another obgyn, or my family doctor, or someone else completely?  I am planning on becoming pregnant soon so should this be done asap?  Or should I just let it go since the ultrasound did not show anything?

My great-grandmother is the only one in my family who has had breast cancer.

Thanks for your input.