The horizontal single wall pipe is leaking water at collar where it connects into triple wall pipe in my attic it started leaking when I started a fire in the stove it is not raining out could the horizontal pipe make this happen if it is not pitched enough its only a 2 foot section

I believe you are getting condensation which will likely stop once the stove is hot. In any case you need to seal the area of the leak. High temperature silicone should work.