Earlier this year my 4YO daughter had a lump on her neck. Apparently when the body is fighting off bugs it sets the lymph system into action and that can inflame the lymph nodes. So it could just be that she is fighting off a cold or some other bug. Because my daughter had a lot of ear infections during the winter (which is rather unusual for 4) my doctor was concerned that there was something else going on- she was tested for leukemia/lymphoma. Apparently lymph nodes also get enlarged when they're fighting any bug off, including cancer. I would say if she has any other wierd symptoms you might want to take her in sooner- like super tired or odd headaches. Around the same time I also found out that lymph nodes can get infected, which just take some antibiotics to take care of- you'd know that because they would be very sensitive to touch.
As it stands for my daughter, the test results came back just fine. :) I'm sure it probably isn't so much a big deal unless there are other symptoms... I just didn't want you to wait until the asthma appointment if there's something else going on. Just the same you might want to call your doctor, depending on how severe her asthma is it may be better to get her on antibiotics... but I really don't know anything about asthma.