Sometimes when we are stressed and tired, it affects even the ability to spell, where the word just doesn't look right. So I would suspect stress and needing to relax, and environmental issues like medications you take. And in the winter it's especially important to change your furnace filter so you have clean air to breathe and good air circulation.
Rebecca, are you having any other problems? Vertigo, numbness, pain, odd sensations? Or weakness in your legs and arms?
Problems with word finding and spelling are common with MS, but at this point it's not conclusive - you'll need to see a neurologist and be evaluated.