Welcome to the MedHelp forum!
Platelet count is often increased in inflammatory conditions like rheumatoid arthritis. It is also increased in some bone marrow disorders like myeloproliferative disorders.Myeloproliferative disorders include: polycythemia vera, essential thrombocytosis, primary or idiopathic myelofibrosis (also known as myelosclerosis) and chronic myelogenous leukemia (CML). Dehydration too is a common cause.
This needs to be followed up with a hematologist.
Hope this helps. Take care!