post whipple strange labs

My Mom (65) had her whipple 5 weeks ago. She's already done 6 rounds of chemo. Her CA-19 was 416 when diagnosed and 110 right before the whipple.
Here we are 5 weeks later for her check up with oncology and her CA-19 went up to 113. We thought that was SUPER strange. Her CT showed nothing except a new "micronodule" on her lung that wasn't there before, but the report says "no evidence of metastasis".
We didn't get a chance to discuss results with her oncologist because they whisked Mom off to a brain MRI due to increasingly bad brain fog and balance (gets worse by the week, even since whipple). Brain scan showed "extensive white matter signal abnormalities" but no growths.
Finally...my question. Why would CA-19 stay about the same? The tumor is gone...I would think the number would have drastically gone down.