Spot on mammogram, unable to locate with ultrasound

I recently turned 40 and had my first screening mammogram. The results came back that there was an unidentifiable mass in my right breast. I went back for a diagnostic mammo. The spot was still there, so they sent me down for an ultrasound. The tech wasn't able to find anything, so the radiologist came in. He was also not able to locate the trouble spot with the sonogram. He said they weren't sure, but it was probably just a lymph node and they'd do another mammogram in 6 months to be sure nothing has changed. Is it normal for something to show up on the mammogram, then the radiologist not be able to find it with ultrasound?

[Cleveland Clinic]

Dear tawfoster:  It is entirely possible for something to show up on mammogram and not on ultrasound.  Ultrasound is best at differentiating between cystic (fluid filled) and solid masses.  When something shows up on one image but not another, the question to consider is the level of suspicion and whether it should be biopsied or reimaged at some shorter interval.  You may want to seek the opinion of a breast specialist (often affiliated with large academic medical centers) who can help you sort through this situation.  You will need copies of your films and reports.