Gilmores Groin will not normally cause abdominal pain. It is basically the tearing of the adductor longus tendon off the pubic bone, so this is where the pain would be. It should be more painful during hip rotation in standing than in sitting as more stability is needed in the pelvis when weight bearing.

I understand what the physio is saying about flat feet, but why has it suddenly come on now if that were the case. If you want to look into this further a podiatrist is the person to see, but personally don't think it will solve your problem.