vet call

I had to call the vet today. Dave was breathing funny last night. I wouldn't consider it labored, but definitely a little heavier than usual. He also threw up a few times last night. I'm hoping that it's due to the windows being open and his allergies. However, when I talked to the vet, she asked me to keep an eye on his lymph nodes and call if I notice any change. I'm nervous... They still feel the same to me. I don't quite know exactly what I'm looking for, but, if it's what I think I'm (barely) feeling, they seem okay.