Some kids are sensitive to touch and can experience unexpected bumps as painful. This can be particularly true when they are in what to them is a stressful situation like school. Certain types of sensory play can help desensitize them - e.g., swimming, ball pits, lots of baths, light and heavy massage, bounce/trampoline work. The opposite is also true - kids who lack sensistivity can be trained by again exposing them in a supported environment to similar things. The brain can rewire itself on this score.
You can also at the same time work on cognition - that is a different area of the brain - and help them cognitively distinguish the difference between a mild bump and a real injury. I find with my child cognition and sensory work taken together usually help things like this.
Also, when you are sick or tired, do the normal bumps aof life hurt a little more. Same thing with kids - when they are hungry, tired or stressed - the bumps of life will hurt more.