Hi,
A shadow seen on Mammography could mean a lot of things and does not necessarily point to cancer. The fact that you are being called back is very common. Sometimes is just the skin that has folded over on itself causing a density/shadow to appear. A spot compression mammogram is a type of mammogram in which only a small area of the breast is compressed to achieve better imaging of a particular region of the breast. This second test will often smooth out the tissue and the "density or shadow" will hopefully disappear. If it does not disappear an Ultrasound is always needed to see things more clearly.
I know that it's very stressing to be called back for additional tests, but try not panic, most often things turn up to be alright.
Wishing you all the best and good luck...