I have ITP - where my immune system sometimes attacks my platelets. Everyone is different in what affects platelet counts, but I would say your hormone replacement therapy could be the culprit. But it could also be a virus or infection - sometimes platelets rebound on their own. 52,000 isn't anything to panic about, but you certainly want to keep an eye on it.
It's highly doubtful the diet coke would lower your count. I drink tons of Tab, and my drs know it, and they said it's not a problem.