Hyponatremia causes

Had a patient admitted to me today, 80 yof..history of SIADH. Sodium in the AM was 114, and was 118 on admission when i drew it. We were ordered to have a CVL placed, which we did, and start 3% saline at 10 cc/hr. Patient had q 1 hour serum sodium checks. Drew sodium from CVL after placement confirmed, before starting saline. saline was started while awaiting results. Came back at 127. Physician was there, and ordered D5W at 30 cc an hour, and stop saline. Redraw in an hour. Drew sodium before starting D5, and right after ending saline. Resulted back at 116...my question is, what do you think could cause such a yo yo sodium level (other than lab error, which we are leaning toward). When i left, they were still awaiting the latest levels. All fluids have been stopped, with orders to continue q 1 hour checks, and call physician with every result (which i know is important, but it seems like we did nothing but draw blood all day..as soon as we would get a result and call her, get a new order, we were drawing another level, only to have to get another order). I would just like to be able to offer some suggestions or insight that maybe we arent seeing. Thanks guys!!