I think it's a very good thing to ask the question. Kyle is right in this sense: everyone has down days. Too many of them in a row, though, can be a sign of a problem, and from what I recall reading in one of the Health Pages on here, MS is known to PHYSICALLY affect mood, as well as give its' special emotional burden to those who have to deal with it.
The good news is that depression is treatable, no matter its' etiology. Keep tabs on yourself, and ask those closest to you to watch for signs. Talk to the doc, if you think there's a possible issue.
I think we all have those days. Even without having MS in the mix everyone has down days. When you add MS I think it's even more "normal" to have days where life seems tiresome. I don't think it's bona fide depression.
Kyle