could be a cold , however I noticed a lot of controversial about saying that it is "just " a cold and that such thing does not exist , it does but it will always need treatment to prevent
A your entire flock to get it and because it is going to be either worse or pass by , but either way :
B these conclusions can not be made without being certain what the cause of the sneezing is , so to put it in other words it 'could ' be a cold , but it could also be the first symptom of something different and therefor just a check up by the vet is recommanded to have knowledge ,
to answer your question here : these things are always something to take seriously indeed , worried ? no , concerned yes .
Put her apart from your other girls for all safety untill you have monitored her behavior and sneezes and untill you got closure for this before you risk the life of your other girls .
well you might wanna do this , it is ofcourse always your decision and not many go to a vet but if you have a big flock I guess it would be the best .
Any sneezing hen here gets quarantained straight away , even when it is just a random sneeze .
If you do not see any snotty nose within a couple of days she is likely to be just sick as in we get those sneezes but I would not take bets on this .
Good luck !