Ok, so that rules out your long acting insulin.
So could be the insulin from lunch causing you to go hypo or a physical activity. Your job does sound quite physical.
Four questions:
What time do you have lunch?
Are your pre and post lunch readings consistent?
Is there a specific activity causing your hypos?
What insulin are you on?
Depending on the answers it could be that you either need to drop your lunch insulin slightly, or have a snack before the activity.