Before you actually call the walmart mastercard security services department,for if you suspect someone else using your credit card, you should always try to remember where you last used your credit card. For example, i recently used my credit card at a travel agency and i realized that i wasn't familiar with one of the transaction showing a different amount. i was feeling worried and i immediately called up walmart mastercard customer services and they were about to cancel my credit card and issued me a new one with a different credit card number. Good thing i stopped them from cancelling my credit card because i had some pre-authorization done on that credit card. Cancelling the current credit card would mean having to call all those company where i had pre-authorization done such as phone bills, hydro bills, gas bills and car insurance.
I was recommended to first call the travel agency and ask them if they might have charged me those extra amount, if not then i can call walmart mastercard and tell them those transaction was unknown and fraud which they will do an independant investigation. If they deemed it that the transaction was indeed a fraudulent one, then walmart mastercard will refund me the credit amount. However, in my case, it turned out that my travel agency had charged me the amount and they actually split it into two seperate transaction under different company name, and the actual total amount was correct. So i ended up not having to call walmart mastercard to dispute the transaction.
On the other hand, if you did really lost your credit card and some one else had actually used your credit card and you could see those transaction on your online account, then you can simply call the walmart mastercard customer service number which is 18883316133. Tell them that you lost your credit card and that you can see someone had used your credit card. They will open up an investigation and if they do indeed find those transactions to be fraudulent, they will refund you every penny in those transactions and they will also send you a new credit card in the physical mail. In that case, if you do have any pre-authorized transaction with other company such as phone bills, gas bills, hydro bills, or insurance, you will need to inform them of your new credit card number.