Taking a few days is normal....frustrating and anxiety-ridden, but normal. Sometimes films have to be sent out to be read then they have to dictate reports, have them transcribed and sent back to the clinic. The ultrtasound is pretty standard. In my case, the lump was actually not seen on the mammogram. The ultrasound gives them another look at the area. Even if the lump looks like nothing, an ultrasound can help determine better about the size, location, density, etc.
Sit tight and try to keep busy this weekend, you will hopefully get the results early next week.