What Causes Swelling And Pain Around Knee Post Torn ACL Repair Surgery?

my son had knee surgery in November from a torn acl, meniscus, and some chipped cartlage. he s been rehabbing since November. he hasn t had any pain since 3 days post op. he was given the green light to begin training w/ his soccer team but no games or scrimmages until mid may. yesterday his knee hurt him a little after practice. today it s a little swollen. he s freaking out. we have an apt scheduled for Tuesday. any thoughts? he didn t fall or misstep. I think perhaps it s sore from over use