From what you have described, I can see why you are concerned.  I would start with your child's teacher.  Ask her what she has observed.  In many schools, you as the parent can request a screening.  What this means is that the school will test to see if there is a possibility of a learning problem.  If there is, then further testing will need to take place.  Ask them also test her hearing and vision.  You want to rule out as many factors as possible when trying to find the cause of her slowing down.  The results might shed a little more light on as to what is happening.  If she does qualify for additional service, having her start at an earlier age can make all the difference.

Note that testing done on the school level can take several months.  You can have it done privately, but that can be costly.  However, check with your health insurance as it might be covered under your policy.  In the meantime, keep working with her.