You may be right when you say that these feelings are a signal that it's time for you to make some more changes. Sadness can help ground us and keep us from a superficial happiness that is really just glossing over real hurts.
Sometimes I withdraw and go inside myself and feel like I'm just trying to conserve energy, like a plant in winter becoming cold, dark and dormant. I think you're right when you said it's just being alive and that sometimes we're up and sometimes we're down. I find that if I can just be with this sadness without worrying about it too much but taking some action, such as trying to meet new people as you suggested, the winter always passes.