What could be wrong with my standard curve?

I ran samples that have sodium chloride in them. I want to calculate though how much sodium chloride (in this case, in mM) I have in each sample.

To do this, I made up a series of sodium chloride standards. I measure the conductivity of everything – the standards and the samples.

I make a graph of the conductivity measurements I got for the standards and use the y=mx+b equation to plug in the conductance results I got from the samples and thus flesh out what sodium chloride concentration I got in there in mM.

But something is wrong with the equation and how it’s calculating the concentration.

For example:
The LOWEST value I have from my standards is a 3.63 in uS for a conductance reading. (This is water – my zero point). The LOWEST value I got from my samples is a 4.31. How is it possible that when I plug in that 4.31 in the equation to get my x value I get a negative number? It doesn’t make sense – I don’t get what’s wrong with my standard curve.