This.
Once you report a problem to HR they go into investigation mode but not on your behalf, against you. HR are employed by the company to keep the status quo that the company wants. If you report a big problem they are not going to go after upper management, easier to blame the victim. Then they will hit you with warnings and PIPs down the line (not straight away too suspicious).
HR like all people in the company are employed by the boss, they do what they are told, if they don't they'll be fired for someone who toes the line.
If it's not major, drop it. If it is, an employment lawyer like suggested above. But always remember that HR are not there for you .