vector.push_back() crashes

I have a problem while working with vectors of a class. I have tried whittling away at the code as much as possible, leaving me with the code below. Yet, after doing this, I still cannot figure out why my program is crashing. I come from a world of VBA and my C++ debugging skills are poor to say the least and I apologize for that up front. Any guidance to improve my ability here will be gladly accepted.

My class:

class Tester {
public:

    int varA;
    int varB;

    Tester() {
        varA = 1;
        varB = 1;
    }

    Tester(Tester& P1, Tester& P2) {
        varA = P1.varA + P2.varA;
        varB = P1.varB + P2.varB;
    }

    Tester(const Tester &Source) {
        varA = Source.varA;
        varB = Source.varB;
    }

};

My test mains:
Does not work:

int main() {
std::vector < Tester > BreakIt;

    for (int i = 0; i < 2500; i++) {
        Tester newTester;
        BreakIt.push_back(newTester);
        Tester& tempTester = BreakIt.at(0);
        for (int j = 0; j < 4; j++) {
            BreakIt.push_back(Tester(newTester, tempTester)); //This is where it crashes.
        }
    }

return 0;
}

Does work:

int main() {
std::vector < Tester > BreakIt;

    Tester newTester;
    BreakIt.push_back(newTester);

    for (int i = 0; i < 2500; i++) {
        Tester& tempTester = BreakIt.at(0);
        for (int j = 0; j < 4; j++) {
            BreakIt.push_back(Tester(newTester, tempTester));
        }
    }

return 0;
}

Does work:

int main() {
std::vector < Tester > BreakIt;

    for (int i = 0; i < 2500; i++) {
        Tester newTester;
        BreakIt.push_back(newTester);
        Tester& tempTester = BreakIt.at(0);
    }

return 0;
}

The push_back() line that breaks does run a few times before t