Do you have quite an arch in the lower back? For example, if you lie on your back on the floor and bend your legs so that your feet are apart but flat on the ground with you knees pointing towards the ceiling, does your low back lie flat on the floor or does is your back still arched? If so the common reason for this is: tight low back extensor muscles and weak lower abdominal muscles. The weak lower abdominal muscles are not strengthened by the deep muscle training you have just been doing. The weak muscles cause your stomach to stick out and the tight low back muscles also reinforce this posture.
If I am right (and I may not be) and this is the case with you, there is a good strengthening program of the lower abdominals you can do and if you persist in time you can correct the postural fault. But you really have to tone up the muscles so flatten your aabdomen and so they provide some adequate resistance to the low back extensors.
The exercise is not that complicated but it is quite precise. Also, as you have just had an episode of low back pain I would seek some professional advice rather than just go ahead and do it. It is also a bit hard to describe over the Internet. Ask if your physio does movement impairment syndrome diagnosis and management (Shirley sahrmann is the method author). it is a different approach to what you underwent which is Queensland university approach. or email the Queenland APA and ask who in your area practices this way.