How tall are you and what kind of Hennessy Hammock do you have? My first hammock was a Hennessy Expedition. I'm 5'11", and though the Expedition didn't bother my knees (even my rebuilt left knee), the shortness of the hammock caused calf cramps, especially in the left calf.
I bought a longer hammock, a BIAS Weight Weenie, and haven't had a cramp since. I'd second trying the longer hammock, which should give you a flatter lay.