Why Did I Snap at Wal-Mart?

So tonight we were at wal-mart and I NEVER EVER by myself anything but tonight I thought I would treat myself to a few new clothes.  There were shorts on clearance for $1 per a pair so I grabbed 3 pairs. I know right I was going to spend a whole $3 on my self LMBO! Anyways while checking out I was paying attention to the prices and all three pairs rang up at $10 a piece so I politely said "Ma'am those are suppose to be a dollar" and what happened next was something I have NEVER experienced in any store...... This woman said very rudely "NO THEY DIDN'T YOU NEED TO CHILL"!!!! as she is putting her hand in my face, yes folks she put her hand in my face while being snappy!!! What happened next was not the person I am, but I couldn't handle her being rude but the hand to the face SET ME WAY OVER THE EDGE!!! The bitch switch went on and I ripped her a new one, my husband, kids and 20 people in line behind me all went silent while I proceeded to tell her that if she did not get her hand out of my face I would slap the stupid right out of her in the middle of that damn store, i do not come to spend my money at wal-mart to get treated like a piece of shit and if she really wanted to get smart with me I would have her job and walk out of the store with everything in my basket for FREE! She then proceeded to tell me to SHUT UP at this point my blood was boiling and whatever came out of my mouth after that I was to damn mad to remember, but I will say this, the managers were pulling her off the register as I was going to customer service to PROVE they rang up wrong.  My husband told me to go to the car that he would deal with it! 45 minutes later he came out with everything I bought ($172 worth of stuff) with a nice big refund of $50.  Why $50 I am not sure why nor am I going to ask why but I will NEVER SHOP at that wal-mart AGAIN!  Was I in the wrong for snapping on her, um NO I do not think so!