There are at least a couple of possibilities, here are the two most likely:

1. This early on the pain may be due to laxity, or "looseness," of the sacroiliac joint. The hormones that cause your ligaments to soften and stretch easily are secreted very early in pregnancy. Hence, many women suffer back pain long before they experience weight gain of their own bodies or the growing fetus. Many experience sharp pain on one side (often in the buttock or hip) with changing position in bed, going from lying down to sitting or vice versa, or climbing stairs. And sometimes the pain mysteriously goes away for awhile only to suddenly return when you least expect it.
2. With your previous back history, however, you may be experiencing a further deterioration or worsening of the herniated disc and impingement of a nerve giving you sciatica.

A chiropractor may be wisely hesitant to manipulate your spine with your back history and current pregnancy.

Regarding the physical therapists you consulted, you didn't mention whether they were specialists in physical therapy for women, specifically, pregnant women. Unfortunately, not all physical therapists know how to treat back pain in pregnancy. I would consult another physical therapist who is a specialist. You can ask physical therapy clinics in your area if they have a specialist on staff. If you are unable to find one this way, contact the Women's Health Section of the American Physical Therapy Association, PO Box 327, Alexandria, VA 22313, for information. You need a thorough evaluation by physical therapist specialist to determine the cause of your back pain and subsequent treatment. Further testing may be indicated to rule out a herniated disc.

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