Just got hired as an overnight stocker at a grocery store, what can I expect is it easy, hard, pain the ass?

Do what the boss says, don't take any shit, and give it your best. Do that and it will be a great job.

You have to move fast and be accurate. It can be backbreaking. It can also be fun with the right crew and manager. Just do what you're told, don't be a slacker and keep your nose clean. You'll do fine. Jobs like this in most chains and even small stores can be a gateway to better positions if you don't screw up.

Did it ever occur to ask your employer exactly what was expected of you? Why would you accept a job when you have no clue what it entails?

Didn't your employer explain your job to you when you had your interview? A stocker is going to be a lot of hard physical work though. If you have any questions about this job .. ask people where you'll be working. We have no idea what you'll be doing.

Best to ask co-workers for specifics, but there may be challenges with adjusting to day and night with work days and off days. Good luck with the job!