I would guess if the 'valgus' test was negative (for MCL), it might be the pes anserine insertion of the adductor longus muscle, because it is brought on with muscle activation, and that is the main adductor that crosses the knee joint.
If its been that long, you can probably start doing theraband stuff for it without damage, but you could also go see a physio, and have some transverse frictions/ultrasound and also a PROPER DIAGNOSIS haha.
Take into consideration I haven't seen your knee at all.
Cheers, good luck!