Sources at the Lynwood Correctional Facility explain the problem to TMZ this way ... inmates can order food and drinks from the commissary on Mondays, and pick it up on Tuesdays. Lindsay didn't check in until Tuesday, and the jailers are making her wait until next Monday before she can order anything.
The girl wants a drink of water, but the only thing that's available -- as one source puts it -- is "water from that nasty sink in her cell." Lindsay just can't stomach it.
So on Thursday, Lindsay asked jailers several times for water, but they wouldn't budge. One jailer told her, "Just drink the milk from the day before."
Apparently, there's a vending machine somewhere in the jail, but no one has offered her the option.