Feel weakness in my legs 5 weeks after fasciotomy, should I be concerned?

Not sure. It is hard to tell if you had both legs involved with fasciotomy or just one. There may have been nerves disturbed that will need more time to heal. If you had any muscle damage from the high pressure in the compartments (ischemia), this may also need time to resolve and heal. I would recommend a good eval by a physical therapist and let them recommend a therapy plan to help you.