During pregnancy it is fairly common to get a split in the rectus abdominis muscle into right and left halves, this is called Diastasis Recti. It is important that you fix this before you start a regular yoga practise or exercise regime as movements that stretch the abs, such as back bends and crunches can make the split worse. When the abs are split it can cause problems in the lower back, hips and pelvic floor due to an instability in the core. When the condition is very severe it can even lead to hernias and incontinence.
The trick is to do little and often, which can be very frustrating if you are used to being very active but you need to be strong in a different way, and repair before you bebuild.
I have created a short video of exercises that can help to knit the muscles back together, I would recommend doing the video once a day, along with regular pelvis floor exercises.
If you do think you have Diastasis Recti please go and see your midwife or Dr.