C++ cerr behaves strange?

I am writing a bash like program which takes commands and waits for another after it handled the input like this:

my_bash> inc
my_bash> bash error
my_bash> 

In case of an error I was printing to cout and everything was fine, when I changed that to cerr my program behaves really strange:

cerr << "bash error: cd: OLDPWD not set" << endl;

After the error message gets printed my program doesn't print another line of my_bash>, and I need to press enter for the other line to show.

like this:

my_bash> inc
my_bash> bash error (in red)
| (cursor here)

then I press enter and get:

my_bash> inc
my_bash> bash error (in red)
my_bash>

what is causing that?

In main.cpp I have:

while(true) {
    std::cout << "my_bash> ";
    std::string cmd_line;
    std::getline(std::cin, cmd_line);
    smash.executeCommand(cmd_line.c_str());
}