Agree with what Daniellabella said - mine (FWIW I had two over a six year period) always ended up a bit further down from where they started but they only tended to move in the first 6-8 weeks after that they were fairly stable in their location.

So long as you can feel it (the entire rod) under your arm you should be right but it doesn't hurt to have it checked out if you are concerned by how much it has moved.