You're as much a part of nature as other living and growing things. Nature follows a pattern, people seem to be the only ones who try and fight it. Fighting the blues can sometimes take more energy than just allowing yourself to have a bad day. Don't fight yourself, don't berate yourself for feeling sad or low, it happens. Moods are like the weather.
Some days are sunny and warm and some are gloomy and rainy.
You can't fight the elements, don't waste energy fighting yourself. Of course if you're feeling down for more than a few weeks and have lost interest in most things that used to interest you it would be a good idea to see a professional. Otherwise, be good to yourself. Pamper yourself as only you can. Take good care of yourself.