By giving you the opportunity to admit use they (a) obtain some confessions to use which might not show up in the test, and (b) give you the opportunity to "fess up" before a test demonstrates a probation violation - that's what they call a "double-edged sword". If you're going to be caught in a lie, telling the truth to your probation officer is the better approach.

Some probation officers would give you a warning; others would commence probation violation proceedings. I would suggest consulting a local criminal defense lawyer to determine what your probation officer is likely to do, and your best course of action.