The red bumps on your ankles sound to me like one of two things. Either 1) they are from chiggers, microscopic little arthropods (I believe it's actually a mite) that burrow into your skin and cause inflammation in the form of little red bumps; or 2) irritating oils from a plant. Poison ivy is not the only plant that causes dermatitis. Depending on the region you're hiking (the Southwestern United States has a lot), there is a wide variety of other plants that can irritate the skin.
If it were heat rash (or miliaria), you'd probably get the red bumps in a more widespread distribution around your body and not just your lower legs. Heat rash is also supposed to cause itching.
My hunch is that it's chiggers.