Dear Susan,
  My understanding is that the calcium in the milk blocks the absortion of
the iron.  During my last pregnancy, I was taking my Ca with my prenatal
vitamin and my iron tested low.  As soon as I split the two up, my iron came
right back up.  This is why, if you read a vitamin bottle, you'll see 100% of
the RDA of most things but only 10-20% of the RDA for calcium.
  I don't know about iron and antibiotics.
Lezlie Densmore
LLLL in MA