What could be causing high platelet count of 900, anaemia after iron infusion 4 weeks ago and two blood transfusions two weeks ago. ?

Thrombocytosis: Thrombocytosis, elevated platelet count, could be caused by overproduction of platelet precursors in the bone marrow. But in your situation the iron deficiency is likely the cause because elevated platelet counts are often seen until the iron normalizes. It is concerning that you require iron infusion and blood transfusions, so you need to determine why you are so anemic.

Bone marrow problem: Two possibilities: 1. You have iron-deficiency (why? Are you having GI bleeding? Gastric bypass? ) and your platelets are high as an inflammatory response, or 2. You have essential thrombocytosis or another bone marrow problem. Given your age, my vote is for #1. See a hematologist for more answers!