Concussion 5 days ago. Rested & avoided cognitive activity since injury. Need to return to work, but get headaches after 40 minutes reading/computing?

Don't overdo it. Concussion injuries recover in variable time frames. You probably need just some more rest time. Understanding how long it takes to develop symptoms is helpful, as you should anticipate but not push the time to headaches and take breaks frequently. You will hear that you should avoid this type of activity; however, if you can tolerate periods of time without symptoms, you are not inducing harm.
Be evaluated. Persistent headache need to be evaluated by a phsyican. Be seen immediately.