Walmart - Credit Card Review

I sigend up to get a Walmart credit card a few months ago and had no problem in attaining one. Though we really didn't want another credit card I thought it would be helpful just use it for the Murphy's gas station the on Walmart property. You get 3 cents off at the pump with the card. I figured if we only use for gas and pay it off at the end of the month it would be okay. You can get a Walmart gift card and put what ever amount of gas you want on the card an use that but this way I wouldn't forget and could still get the 3 cents off with the cridit card. After having it for only a few months, all of a sudden they lowered my credit limit to $100. There was no real explanation and our credit hadn't change, or if anything, may have improved, since I got the card. Only one time did I use it for something other than gas and that was less than $20 and we did pay off our balance when we got the bill. I almost feel like I am being punished for paying my bill on time an not carrying a balance from month to month. Though it may be inconvenient, if I want the gas discount I will use cash or my debit card & put the amount I want on a gift card.