The egg laying system sometimes causes some discomfort, I think, when it starts up.
But if the duck is otherwise healthy, it's no big deal after a few to several days.
Some concerns - if she is not absorbing enough calcium, that could cause serious trouble. Or, if somehow some egg material gets loose inside her, that can cause infections - again, serious trouble.
I have had ducks have a little trouble with laying either beginning or stopping. But with good nutrition, extra calcium, a little TLC (lukewarm tub baths, no soap), they generally come around.
If not, I would seek a vet's help.
There are a few things that can go on with the oviducts, that can cause problems aside from what I mentioned - a vet might be able to figure out some of that.
Majestic Waterfowl has a symptom chart that I look at from time to time
http://www.majesticwaterfowl.org/diagnostic chart.htm
You said she seems light - that worries me.