Llyr, I'm no expert at all, and know very little about this topic. But I wonder if it's the mothering energy, that protective instinct coming out and not knowing where to go? I know sometimes if I miss one of my kids, or have sent them somewhere new (such as a camp or something where I'm not seeing them daily), I feel a little lost. I don't mean that I don't know what to do with my time or anything like that. I mean, it's my job to look after them and protect them on a daily basis, and when I send them off somewhere, it's a different way to do the parenting job. I think as parents, we feel more secure in what we can do for our kids when they are physically with us. Or at least, if they're at school, they're not far away.
But it can be unsettling to not feel in control of their safety and protection. I think the energy is natural and valid, but maybe it needs to you guide it? So, perhaps you could do something to send that energy to your child, somehow. Like I said, I'm not well versed on these things, but maybe you, as a mother, still need to express that protective instinct, and it's hard to do that when he's not there, so this could be how it came out.