Postural correction is my specialty, but with all due respect, this could be a number of things. But no doubt you'd see improvements from these:
- Stretch your hip flexors regularly. (
http://www.exrx.net/Str.../KneelingHipFlexor.html)
- Stretch your piriformis. (
http://www.drbackman.com/piriformis-muscle-stretch.htm)
- Ween your way into doing Deadlifts and more functional core movements if you haven't already, as in, movements which stabilize your entire core, such as prone bridges (look them on google/youtube, I cbf linking).
Also, with stretches, make sure you know where the muscles are, so you know what's supposed to feel a stretch.
And in response to your question about building strength in the lower rectus/tranverse, those functional core movements are ideal as they work the entire core.
It's suggested that the main job of the abs are to decrease the pressure put on your spine during hip flexion, unnecessary which is always there and always detrimental under high pressure on the facet joints, hence why I firmly and professionally believe that core stabilization (those static contractions like the Prone Bridge) are much more beneficial for building abs and core strength than dynamic core exercises ever could be, as they put less pressure on the facet joints, they're beneficial to everyday situations where we require core stabilization, they strengthen the whole thing (not just target one area) and they break down the muscle tissue just as efficiently if not moreso.
Hope that helps.