Have you looked into PUPPP (I gather it could happen post-pregnancy as well)? Here's from babycenter.com (just the first site I got to when I googled):
"pruritic urticarial papules and plaques of pregnancy (PUPPP) or polymorphic eruption of pregnancy.
PUPPP usually begins in the third trimester and is more common among women carrying twins and those having their first baby. The eruptions usually show up first on the abdomen around or in stretch marks (if you have any) and may spread to your thighs, buttocks, and arms. PUPPP is harmless for you and your baby, but it can itch like crazy!
Your doctor or midwife will want to see you for a diagnosis and will probably prescribe a topical ointment to give you some relief. She may also recommend an antihistamine. In severe cases, you may need a course of oral steroids.
PUPPP usually disappears within a few days after delivery, although it sometimes persists for several weeks. (In rare cases, it may even begin after you give birth.) Fortunately, it seldom appears again in subsequent pregnancies."
Just another thought. Best bet, I think is to ask your doctor or get a referral for a dermatologist. Good luck!