So, because you were there simply to "check on something" you should be serviced first? How about the other people who were in front of you in line? When you go to the DMV just to change an address, or get a new picture, are you in and out in 5 minutes? No, you take your number, wait forever, and when it's your turn, it's your turn.
The way you put it, since people are there to order a product, they should have to wait until after you, even though they were there first? Though it, perhaps, wasn't the most polite thing to say, she's correct, even children learn patience. Perhaps you needed that slap in the face of a remark. After all, you admit she wasn't rude, in any way, up until you made a snide, out of line comment about having to wait your turn... Good for her. Though you obviously learned nothing, because only an ### complains about having to wait for patrons, who arrived, and signed in, and waited their respective turns, to be helped.
Oh, and perhaps she did 'just check for ya quick', and then 10 other people wanted the same... well, then that person who was there to be helped, after waiting, now has to be further inconvenienced. How would you like it if she were selling you something and had to walk away from you, numerous times, "just to check something quick" for people.
Learn some patience...