Mass near chest wall seen on mammo but NOT U/S, any ideas??

[LoreleiNL]

I just had a diagnostic mammo and U/S following a routine screening (50 yrs old, no history of bc, no symptons). They found a suspicious mass near chest wall/inconclusive, and could not find anything at all on the U/S. now am scheduled for MRI next week. They say the MRI is still just to rule out anything serious - it could be lymph tissue? No reason to be concerned, they say.... Anyone had a similar experience, or could shed some light on the possible findings? I'm freaking out a bit here!