Increase in synovial fluid (synovial edema) puts pressure on nerves. Similar to the way fluid spillover into the carpal tunnel can cause tingling or numbness in hands due to GH's overall sodium retention abilities.
It should subside in time. If it doesn't, GH may not be a good fit for you. Low-impact continuous joint flexion (e.g. light walking or pedalling) will ease the stiffness but it will return if you're not persistent. But again, it usually subsides altogether in time..