Swollen lymph node is not a reason for ER visit. You should have gone to see a pediatrician, ER doctors are trained to deal with emergencies and not minor health issues. Lymph nodes can get enlarged due to an infection the child's body is trying to fight. One condition in particular, called tonsilitis when lymph nodes swell on both sides of the neck. The swelling will resolve as soon as the body overcomes the infection. Lymph nodes can swell when child is having any infection or inflamation of throat, runny nose, ear infections, other upper respiratory issues.
In your situation, I would follow up with pediatrician, a lot of infections resolve on their own, thanks to our great immune system.
As for difficulty breathing, the symptoms to look for are those that show that your child is working hard to breathe, look at the chest, does it look like she is trying to suck the air in? Is she pale, or dusky?, limp, sweaty, clammy , breathing too fast consistently or too slow - those are the sighns of an emergency, if the child just makes some funny upper airway noises or has stuffy nose - there is no need to rush to ER. Human body can handle a fair bit of congestion before it becomes a problem.
Good luck,
Hope your little one feels better.