I had an ultrasound because my mammogram found cysts. What that overtesting?

Not overtesting. Mammography cannot determine if a nodule or nodular density is a solid mass or a cyst. Ultrasound is necessary to make that distinction.
No. No, that's the way it's supposed to happen. A mammogram can't determine whether a mass is solid or a cyst. An ultrasound can. Cysts are benign, solid masses may not be. So this is a hugely important piece of information that ultrasound provides.
Absolutely not. The mammogram is valuable in detecting things that cannot be felt on exam. If there is a lump or a finding on the mammogram, the ultrasound is valuable in determining if the lump is solid or cystic (filled with fluid). The tests work well together. Solid and cystic lumps may be managed differently. This is an important distinction to make.