Lifting after cortisone shot in thumb?

I just got back from getting a steroid shot to treat trigger thumb. It wasn't planned and then the Dr. said not to lift anything heavier than 2 lbs. for 48 hours. She said I need to give the steroid time to be absorbed into the tissue.
She didn't say I couldn't use my hand and didn't even encourage the use of a thumb brace. She knows I stock merchandise and wasn't concerned about my using it... just about lifting anything heavy. That seems odd to me. I have to go to work. I've only had the job for 5 weeks and this month they have cut back hours drastically. I'm debating going in and not showing them my letter and seeing if I can get through my shift using mostly one hand. If that doesn't work, I can't very well pull out the letter midshift. Or I could just show them the letter in the beginning and maybe they will send me home... and cut my hours more than they've been cut.
The thumb isn't any more sore than normal. Is it possible that lifting light weights to support the "good" hand will cause the steroid shot not to be effective?
What experience have you had with these shots?

Thanks for your reply. I did show the letter as soon as I walked in. Coincidentally, they had me assigned to location that was more security than stocking and it worked out very well (other than it was terribly boring!). I'm glad I didn't have to overwork the joint and now it will either heal or not.