Preschoolers are not great at multitasking. They are incredibly smart, have amazing imaginations and are curious beyond belief, but when it comes to doing a few major things at once, it can be overwhelming. Adjusting to preschool is tops on the list of "things that can cause a child stress." And even if your little one loved preschool from the very first day, he can't wait to get out of the car and into the classroom, it is likely that you may see some regression in other parts of his life.
Generally, as a child learns a new skill or task, other areas will suffer. So while he's at school, getting used to the routine there, making new friends, learning about shapes and colors and maybe even how to write letters, there is a lot going on in his brain. There is lots to remember and take in. It's fun, but it is still different and stressful. Home is safe. It's a constant. It's natural to expect an occasional extra temper tantrum and moodiness.
You can help by being supportive. Lots of hugs and encouraging words. Talking is good too. Ask your child to name his favorite things about preschool. Who are his friends? What does he play outside or at free time? Praise him for doing such a great job at school as well as when he does "big boy" things at home -- maybe he remembered to hang up his coat or put away his toys without being asked. Drawing attention to the good things is a great way to encourage that type of behavior.
The timing is important too. If this is occurring on occasion, say, only when he is tired or after school, chances are this should all sort itself out within a few weeks. If he is acting like this all the time, you may want to consult your the teacher or your pediatrician. If you preschooler is young (under three-and-a-half) or this is the first year of preschool, scaling back hours might help him relax as well. You can increase the time he spends in school slowly, monitoring his actions and behavior as you go.Keep in mind too, that your child may experience another adjustment period after returning to preschool after an absence, like a school or personal vacation. Just wait it out and keep giving him all the support and love that he needs.