C++ adding two doubles

I've been working on a little project, when I have encountered a rounding problem (I think). So, here is my code:

double temp = (Layers[i - 1].GetElement(l, k) * Objects[i - 1].GetElement(k, 1));
sum += temp;

And here are the default values:

-sum = 0

-Layers[i - 1].GetElement(l, k) = 1

-Objects[i - 1].GetElement(k, 1) = 1

Yet, when I run my code, the end value of the is a very small, negative number (like -6.99885e-203). I'm a beginner with c++, and I have never encountered anything like this.

Any help would be appreciated!