I suspect walmart.com may have been compromised. I'll be interested to see if the news reports on this happening to more people in the next few days. I have had them run a full malware scan too, they are clean, this appears to be their online WM account that was accessed and nothing else.
I just assisted local friends of mine, someone in another state ordered a $600 TV through their Walmart online account for store pickup. They noticed it immediately by checking their email(good for them they were still awake), logged into their account and found a large purchase. They cancelled the order, removed payment info and changed the account password, as well as called WM's customer service number.
If you order through walmart.com you may want to at least change your password and/or remove your payment info, in case they have had a security breach of account information. They have done that with WM and their Amazon account for the time being.