I had some work done recently on suspension shocks, springs, mounts changed, discs changed, diff and gear box were off,after all done I "noticed a little problem"

It drives lovely and brakes as it should and everything seems OK until .... I put it in reverse, and while moving back (slow or quick!) when I brake (slow or quick!) I can hear and feel hit from under the car, first time I noticed it I thought I hit something, but noooo...
went back to mechanic, sat him on passenger sit and drove forth and back and forth and back and no problems whatsoever

it seems to me that it appears only when I drive forwards for a little while, and then when I need to park or just revers, it hits again, even if I brake really gentle while moving backwards, it still hits quite hard. Sound from back underneath, may be back chassis.
Any ideas?