C++ double operation giving different result in different condition

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C++

Good Morning,

I developed a Vc++ DLL called by a VB program. The DLL has a function that does some operation on vectors of points(double[3])

now suppose we have two set of point S , T.

If The Vb program calls the function only one time with set of points T i get a result.
If the Vb program calls the function two time, first with set S and then with set T, the result for the set T is different from the result before.

I try to debug and the difference is a product of two double that act differently.
I have two double dx and dy and the function does:

dx2 = dx*dx
dy2 = dy*dy

with the debug strings i get:

if i call only Set T i Get

[2920]point 10-->   dx: 32951.71011630935400000000 dy: 1784.60091076856500000000 dx2: 1085815199.58928420000000000000 dy2:3184800.41071599160000000000 A:1089000000.00000020000000000000   B:0.00065903420232618713  C:-272250000.00000000000000000000  

with the call with set T after the call with set S:

[2920]<pre>point 10-->   dx: 32951.71011630935400000000 dy: 1784.60091076856500000000 dx2: 1085815199.58928390000000000000 dy2:3184800.41071599160000000000 A:1089000000.00000000000000000000   B:0.00065903420232618702  C:-272250000.00000000000000000000  

as you can see dx is the same for both the calls, but the resut dx2 is different.
in final result the difference accumulates for all the points and i get slighlty different value.

Does anybody have an idea of what is happening here?
It seems the after the first call the function is in a different state but how this can affect a product with same operands?
thank you

What I have tried:

I tried to check for memory leaking but i didn't see nothing odd.