S.
It's always hard to understand everything that goes on their little heads. My son is now 2 and my daughter 3, they still go through phases and I think that it's normal, though unsettling for us. When you think about it as adults, we also have those mood changes, fears etc :-)
Most of the time, I understand the reason of all that fussing only after the episode... Frustrating. At your son's age, there is a lot going on in his development, physically (such as walking) and emotionally plus the fact that he cannot speak yet. So my first guess would be that teeth are on their way out (or gum swelling). Second guess separation feeling (this occurs at different ages),I would keep his routine just as it is, and keep reassuring him and comfort him with the same words and hugs and kisses. I personnaly also use homeopathy and Bach flowers, it works well with kids. Hope this helps.