After 3cycles of chemo she had low plt but no transfusions,on the same cbc pmn count was 28000 and after 2days waiting for plts to rise it dropped to 13500,gcsf was administered.why a sudden drop in pmn?she had a cold,could it be post-infectious?

Not unusual: GCSF effects on PMNs are short lived(2-3 days), so the blood counts rise in 24 hours and stay up only for 2-3 days then they return to normal in 4-5 days. Platelet drops can be due to excessive PMN production as well as the continued effect of chemo. I would recheck the platelet count to be sure that they do not drop below 10, 000 which is a count that requires platelet transfusion. Good luck