That is a normal sensation. Think of getting your blood pressure taken. When the cuff is inflated on your arm, occlusion pressure exceeds systolic blood pressure (pressure created by heart contraction). That's means no blood flow is able to travel to the arm. Muscle contraction can do the same thing. If you flex hard enough, the pressure on the blood vessels is greater than your blood pressure, resulting in lack of flow.
It's pretty interesting to think about this. In the case of the heart, it delivers blood to the entire body as it contracts. However, when the heart is contracting, it hinders flow through the coronary arteries to itself so most of the heart's blood supply is delivered during diastole (relaxation of the heart).
The body actually uses this mechanism to the body's benefit through skeletal muscle pumps. It uses the pressure created by muscles to push venous blood back to the heart (via one-way valves). I just threw too much info at you, but I thought I'd put it into perspective.
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