Mammogram after ultrasound

When I got the phone call telling me I needed a mammogram, part of the reason why I was so freaked out was that my research about breast ultrasounds had suggested that ultrasounds are superior to mammograms and are sometimes ordered as follow-ups to mammograms (as opposed to stand-alone tests like I had), so I couldn't fathom why a mammogram would be ordered after an ultrasound. I googled around the idea of mammogram after ultrasound, and couldn't find anything informative - mostly just people saying that it seemed backwards and ultrasound normally comes after mammograms.

I know I've already posted at length about my breast lump diagnosis process and this post basically repeats that information, but with more focus on why the mammogram was applicable after I'd already received the breast ultrasound, and what we learned from the mammogram that we didn't learn from the ultrasound, with the target audience being people who are googling for patient experiences with a mammogram after a breast ultrasound.

Patient profile: 34-year-old female, never been pregnant, no family history or risk factors of breast cancer

Diagnostic process: While conducting a routine breast exam during my annual physical, my doctor noticed an assymetricality in the armpit area of my breasts. He ordered an ultrasound, which found the lymph nodes were enlarged on one side. The mammogram was then ordered to get a better look at the lymph nodes.

My doctor's explanation for the mammogram:  My doctor said that the mammogram can get a better look at the architecture of the lymph nodes than an ultrasound.

The content of the mammogram report: The mammogram report said that there were no malignancies, and also said that my breasts are fibrocystic.  The ultrasound report did comment on either of those things.  The logical conclusion would be that the ultrasound couldn't determine either of those things, but I don't have the medical knowledge to make a definitive declarative statement to this effect.

Diagnosis: enlarged lymph nodes on one side but not the other. I believe this is due to a recent vaccination. I've been instructed to get an ultrasound again in 6 months to make sure they're back to normal.  (I hope the don't follow that ultrasound with another mammogram!)