what you are doing is developing a mental preference of the relaxation and then realizing that this obstructs your outward activity of high concentration.
it might be described as a pitfall along the way.
This is due to the very subtle nature required of repeating the mantra in deep meditation.
it seems that you may not be grounded enough as often relaxation can aid in concentration. so being too relaxed can make you feel a little floaty or unable to steer yourself mentally. perhaps you can find a grounding exercise suitable.