Johni - an LMO does not authorize you to work, only a work permit does. If you started working 6 months before you were authorized to work... oops, that's bad. At best, you cannot count those six months. At worst, you and your employer could have problems. You won't be able to hide it either, as your tax records will show you were working.
Under what circumstances were you in Canada prior to receiving your work permit? I mean, were you on a visitor visa, student permit ... ?
You might consider consulting an immigration lawyer for advice.