Well once the head is born the shoulders are usually at the narrowest part of the pelvis, so they must rotate to fit through the widest part of the pelvis before the body can be born. With SD the rotation doesn't happen (sometimes due to size, position etc) and the shoulder becomes stuck behind the symphysis pubis. Usually McRoberts position helps to dislodge the shoulder and allow the baby to be born but often there is a brachial plexus injury where the shoulder dislodges.
Nobody on here can tell you what happened at the birth, but as it's documented that they saw the shoulder it isn't likely it was SD. Is it written in your notes anywhere about SD? There is also a lot of documentation after this happens.
However it does sound like they were worried that it might be SD and prophylactic McRoberts is often used when the baby seems to become stuck, even if it isn't SD. This happened to me.
If you are unsure about anything I'd definitely recommend talking to a midwife, you can arrange a meeting to go through your notes, ask any questions etc and they will explain it to you. It can be so confusing looking back at your birth and not having the whole - picture - if you know what i mean? Especially when it's not gone to plan or something happens like this. Going through your notes with a professional who can explain it all can really help you understand.
I plan to do this before my having another baby as even over 2 years later I still get flashbacks and I kind of want closure.
Good luck xx
