I've always been nice and flat around my pubic bone but now I'm noticing that I'm developing fat there. Can this be due to a test/estrogen imbalance?

Yes it could be. I notice you are already on Testosterone. Make sure your levels are within normal range and that any symptoms from before supplementation are improving. Don't overlook the obvious in that the pubic area fat could be related to over-all body fat increase if you are eating too much and not exercising. If fat is only certain areas, it could be hormonal.