Last I checked, the Constitution is silent on anything related to stalking. However, things can be in violation of other laws passed by Congress, state legislatures, county governments, or city governments. But that's an entirely separate question as to whether something is unconstitutional.
If someone shouts, "But that violates the Constitution," that has a very specific and narrow meaning and, unless you can cite the exact part of the Constitution it violates, it not unconstitutional.
I'm talking ONLY about the Constitution.
That aside, generally, someone being stalked knows they're being stalked and fears for their safety. If I followed you around all day staying at least 500' away, recorded your every movement, and you didn't even know I was there, then AFAIK, that's not against any law. (If it were, restraining orders wouldn't be a thing.) But, again, this has nothing to do with the Constitution or being in violation thereof.