Actually the terminology there is all messed up – not sure if it was always that way. “Congress” properly refers to both Houses. Members of the Senate are called “Senators” but the language seems to have hit a roadblock when it comes to a parallel nomenclature for a member of the House of Representatives, and for some reason “Congressman” came into use, with no particular care either for what to call a woman in that position, or the fact that technically Senators are also Congressmen (or women).
Both problems are solved by simply calling members of the House “Representatives”, and that’s even their official title, but it just hasn’t caught on in the language. I suspect probably because “Congressman” sounds more lofty, while “representative” sounds like it might be a traveling salesman.