ca19-9 increased and CEA decreased after LAR and during treatment

I was operated by LAR for a stage 3 rectosigmoid tumor. I am on chemo and targeted therapy. After 6 cycles of biweekly chemo, my CEA has declined compared to value before surgery, but the ca 19-9 has increased to higher values than before surgery. I have an appointment with my oncologist next week but i am worried since I am on treatment and my surgery 4 months ago supposedly cleared the disease which was verified with PET scan before i started my chemo. Can anyone clarify this test result for me please?