It is definitely okay to reset the message, which is what the folks at Walmart should have done. The light / message on your dash is nothing more than a timer and distance counter. It sounds like the folks at Walmart did not reset it when they did your oil change. It's easy to do, but the method changes from model to model, so a quick Google search should turn up how to do it for your car. So to be clear the answer to your question is that you don't need to worry about changing your oil again. There isn't a sensor in the oil pan that's detecting anything wrong with the oil.
In general, and this is my preference, not fact of any kind, I have had bad experiences with the cheap oil change places. I now only do them myself and don't let anyone else touch my truck unless it's work outside my ability or tool set. I had one such place nearly hand grenade my engine by not bothering to tighten the oil drain plug. When I pointed it out to the manager on my next trip, his response was to put the longest wrench he could find on it and hang off the end of it. I was lucky he didn't snap it off. I had two words for him: Torque Specification. :) I would consider having a reputable garage do these for you. It costs more, but if you were having a blood transfusion, would you want the best quality, or the lowest cost. Oil is your engines blood. Cheap oil or filters are not a good idea.