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BKIllini, it sounds to me that you may have a partially herniated disc(s) near the pelvis, maybe L5/S1 or one of the ones above it. When you stand up, the bulge may be pointing in a direction that does not aggravate any nerves. But if you sit down improperly, then you feel it. I would definitely go to a clinician and have it checked out. You do not want to work out and have it herniate in which case the recovery would be MUCH longer.