Lump didn't show on sonogram

I'm 29 and just had a sonogram for a hard pea size lump that I've had for about 6 months  1 inch from my armpit.(hasn't changed in size and it doesn't hurt at all) The radiologist showed the pics to a Dr. who said there was nothing there and she told him she could easily feel it and did a second one using a gel pack and still it didn't show. He then said I might want to get a mamogram (mammogram) since my aunt had breast cancer. Isn't that backwards? Why wouldn't it show up on a sonogram when you can definately feel it not too far under the skin? My breast are pretty lumpy anyway, but this one is definately different.

[CCF-RN,MSN-rf]

Dear cherylp:  Sonograms are best at differentiating solid masses from fluid-filled ones.  Since it uses sound waves, the more solid structures appear white, and fluid appears dark gray or black.  Everything else appears gray.  They may have chosen to start with sonogram because young women often have dense tissue, making mammograpy difficult to read.  They may now do a mammogram to see if this lump shows or has calcifications.  If nothing shows on mammogram, and this lump can be felt, a biopsy may be the only way to determine what it is.