Brief Answer:
Thrombocytosis

Detailed Answer:
Hello and welcome,

Since you said your platelet count is elevated, I'll assume that your count was in the the 900,000 platelets per microliter of blood (rather than 900 platelets, which would be a low level).

There are many causes of elevated platelets (thrombocytosis). It can be caused by any of the following:
Blood loss
Allergic reactions
Cancers
Kidney problems
Prolonged/vigorous exercise
Heart attack or a coronary bypass
Infections
Iron deficiency anemia
Past removal of your spleen
Hemolytic anemia - where your body destroys it's platelets
Autoimmune inflammation
Recent surgery
Pancreatitis
A major trauma such as a car accident or burns
Certain medications including:
Epinephrine (eg EpiPen)
Tretinoin - for acne
Vincristine Sulfate - a medicine used in chemo
Heparin - for blood thinning

The first thing to do is to get your test retaken, and a physical exam to check the size of your spleen. Other blood tests to do are to check
Iron level (for anemia)
Inflammatory markers (such as a "sed rate" which indicates inflammation going on somewhere in your body)
And your doctor may want to test for some autoimmune disorders if he/she thinks it is indicated based on your history.

Depending on what these show, your doctor may also want to do a genetic test for "essential thrombocytosis".

And depending on your history and what all of these tests show, down the road your doctor may want to order a bone marrow aspirate to check to see what is going on at the source of where platelets are made.

I hope this information helps.

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