MLAARE -Gibberish Talk

So I hopped on register today,
To help back up,
As we were strangely short handed....O.o Compared to how it's been the past little while, it was rather strange to have so few people.
In any case, I hopped on.
Right as I got to this lady.
She wanted to return some food that her cat wasn't eating, and exchange it for a different bag.
Simple enough.
Or so I thought.
I looked at the bag. "Has this been opened or used?" I asked her.
She gave me this look like I'd just spoken a gibberish to her.
I had to repeat the question.
"I don't know." Was her response.
So I took the bag, and saw that yes, it had indeed had been opened. I couldn't figure out how she wouldn't know though....wouldn't it be obvious if she was the one feeding the cat? Was she the one feeding the pet?
In any case, I returned the food. And asked her "Do you want your receipt back?"
Again, she gave me the look of "you just spoke gibberish to me."
It was then, I decided that I should try and make this as simple as possible. There was only the one bag of food on the receipt so I tossed it.
"Alright, so I need you to sign here." I said gesturing to the touch screen where people would slide their cards to pay.
She pulls out her credit card and moves to slide it.
"No, you need to sign first."
She nods, moving to slide the card.
I stop her. "No you need to sign here on the screen first." I said pointing to the screen.
Again, I apparently only spoke gibberish. It took me and the guy she was with (this woman is an older woman btw) to make her understand that she needed to sign. So she signed, allowing me to go ahead and ring up her other bag of food. So that the money from the returned bag could be put towards the new bag so she could pay the difference.
"Alright, your total is $7.80" -Because the bags weren't the same price.
She spoke as if she hadn't heard me. "So am I going to get the money back for the first bag and then I pay for the second?"
I shook my head. "No, I've already done the transaction together, so you're just paying the difference between the first bag and the second bag."
Ugh. You can guess what her reaction was.
Yah. The look of "You're speaking gibberish to me again." look.
I just wanted to scream by this point!
GAH!
I had to explain it to her multiple times, with the help of the guy she was with.
In order for her to 'understand'
I don't know if she did.
I don't know if she couldn't hear me, or my words just weren't computing or what.
But finally, she slid her card and I was able to tell her to have a Nice Day and go on to the next customer.

Still....it's reactions like that....that make me wonder....am I really speaking gibberish? O.o
How could my words not be understood?
I really have no idea. :S

-Sarnic Dirchi