having just had an operation could imbalance your body enough to set off an allergy you never had before

itchy all over... some meds can cause this, some cause big red itchy patches or small flaky itchy patches. allergies to scents, perfumes, soaps or washing detergents could cause itching, also some talc and dust mites can too! i am fine with Asda talc but Boots talc feels like i am sitting on a pile of pins, really itchy prickly
you could : try drinking camomile tea it is a soother.
try a mild steroid cream like betnovate but use sparingly,
try eurax cream- this stops itching but is not a steroid as far as i know
check your temperature, often allergies raise it a little causing itchyness, if so take paracetamol which will reduce temp and itchyness.
if it is the margarine you ate, it should go in a couple of days after you stop eating it.