That is not completely true about diarrhetics thinning blood. There are a few prescription diarrhetics that can be prescribed for that, but most of them have no influence on blood viscosity. Decreasing the volume of fluids in your body WILL cause thicker blood, as dehydration (and stress), reduce blood plasma volume.
There are 2 things that can cause 'thick' blood. One, your red blood cell count being too high, and two, a problem with your plasma.
Go get some blood work to play it safe. Blood pooling in the legs is never good. It could be the onset of a more serious condition, and it's always best to have a doctor check things out.