Your son may be giving you evidence of a cognitive leap - he loves you and does not like to see you go... and now he can remember that when you leave it is for a while, not just a moment. He will learn to manage those feelings AND the awareness that you will be gone for a while soon. As he grows he is likely to have other leaps of awareness that make separation difficult or (this can be worse for you) really desirable! Wait til he kicks you out... :) On a separate note... If you have a strong instinct that something is amiss I encourage you to delve. Maybe one of his current teachers likes super sedate children best and has yet to fall in love with his active ways (or the reverse). It can be a real issue without being terribly serious - he and his teacher will settle in together in time. But if you investigate a little, you could speed up the process by helping the teacher see it and that it has an impact on your little guy.
Reply to Sudden attachment/abandonment issues?