String problem

[Sanix]

I had to program a string class as exercise. I should provide a method concat.

Whenever I use it the program stops working without showing some error or something else. It just quits and does not process the following lines after I use the concat method.
I played around and figured out that when I change the signature to
MyString& concat(MyString& my);

it works but by copy it does not with the following:
MyString& concat(MyString my);

I don't understand that.

My header file:
#ifndef MYSTRING_H_
#define MYSTRING_H_

class MyString {
private:
	char* characters;
	int size;

	int length(char* c);
public:
	MyString();
	MyString(char* c);
	MyString(const MyString& my);

	virtual ~MyString();

	char* toCString();

	MyString& concat(MyString my);

	int getLength();
};

#endif /* MYSTRING_H_ */


MyString class:

#include <iostream>
#include "MyString.h"

MyString::MyString() {
	//Init char with size 0
	characters = new char[1];
	size = 0;
	characters[0] = '\0';
}

MyString::MyString(const MyString& my) {
	characters = my.characters;
	size = my.size;
}

int MyString::getLength() {
	return size;
}

MyString::MyString(char* c) {
	characters = c;
	size = length(c);
}

char* MyString::toCString() {
	return characters;
}

int MyString::length(char* c) {
	int i = 0;
	while(c[i] != '\0') {
		i++;
	}

	return i++;
}


MyString& MyString::concat(MyString my) {
	char* c = my.toCString();

	int l1 = getLength();
	int l2 = length(c);

	size = l1 + l2;

	char* temp = new char[size+1];	//+1 for \0



	for(int i = 0; i < l1; i++) {
		temp[i] = characters[i];
	}
	for(int i = 0; i < l2; i++) {
		temp[l1+i] = c[i];
	}
	temp[size] = '\0';	//Termination


	//Copy string
	characters = temp;


	return *this;
}



MyString::~MyString() {
	//free memory
	delete[](characters);
}