Used permethrin for 2nd time last sunday. Really really itchy now. Moreso in my groin than before applying. Reinfestation or post scabies?

Depends. Its not uncommon to be itchy after 2 treatments for Scabies. I would suggest a follow up visit with your doctor to reexamine your rash and diagnosis of scabies.
Possible resistence. Resistence to premetherin has been suspected for sometime; If 2 rounds of permetherin have failed (assuming application and disinfection instructions have been followed) then, an alternate treatment may be needed. See you physician for re-evaluation.