Ah, yes. I call these "tender" days, when the little ones just seem upset for reasons that aren't discernable to us, and our little ones are just too young to reveal or understand. On these days, it's really difficult for adults to suss out 'what's wrong', so I just try to be mindful of a few things. I offer more opportunities for cuddly times, stories, and hugs. I give children more space around transitions, which means more time, singing them through the changes of the day ("Su-sie I see it's time to put the toys away, I will help you") and giving them LOTS of support for those transitions which means being both mentally and physically present when we are asking them to do something and offering plenty of assistance, (these are not so much the days to insist a child do it themselves, so find a way to be cheerfully helpful if need be). I offer as much downtime as I can, so that the child isn't tired or upset and being swept up into my world. I also offer unstructured relaxing play when I can: playdough with popsicle sticks and spoons/forks to push into it (which is right for this age and skill level), a bin of warm water and bubbles on the floor on a blanket or a cozy bath. This isn't to spoil the child, but to comfort their upset feelings.
Yes, this is normal. Some children go through phases of this mood, some experience it for a day or so and work through it. What causes it? In my years of working with children (2 and a half were as a teacher in a toddler room), this sort of 'no reason-upset' is very common. If we are in tune with the child and have some connection, we'll be able to see if the child is needing adult empathetic support --some kids want snuggles-- or if the child is actually needing space to play/be alone, and a well-appointed classroom will have some centers or areas that are cozy spaces for just one child. Upset children often have a harder time negotiating peer interactions, so keeping an extra eye out and understanding that even parallel play may be more work for the child than usual helps us to best give the child what they are needing for this day.
Sometimes, this is just a part of growth. I'm always wary of saying "there's something going on at daycare" because parents usually know in their hearts if something isn't right at school. Some children have some of their own existential angst; some see other children more able to do some things than they are, which is frustrating (and motivating). And their developing brains have so much work to do, too. For more on that, check out Margo Sunderland's "The Science of Parenting", which is an easy, great book and gives practical advice for helping our children in working with their emotions.
You could also make a checklist in your head of any big changes your son might have experienced in the last 6 months. Daycare? New sibling or new person living in your house? Did someone leave the home? Pet? Teacher turnover at daycare, or a new child in their group? Nothing specifically "negative" has to happen for your child to feel some upset regarding change.
I do say, that if your gut suggests a quick once-over by the doctor, it never hurts. However, if you think it's just a 'mood', give him plenty of love and support, and he'll just have to work through it. It's okay. He'll be fine--- and then something else will come along. Isn't that the way it is with kiddos?!