Sounds like a strain and overuse injury. I recommend a professional fit that can identify posture and rotational abnormalities. From what you are describing it sounds like a muscle strain with maybe a inflammation/sprain of the medial collateral ligament. This happens a fair bit due to overuse and overreaching (in terms of level of fitness and attempted effort) which tends to lead to poor body mechanics as your body recruits other muscles and structures to help out accomplishing what you are demanding of it. While I cannot diagnose you, and nothing here should be taken as medical advise, these injuries tend to resolve with slow physical therapy at home (sometimes within a week of rest and light activity the symptoms are gone). Then gradually building up your fitness and body to achieve those goals. Stretching can help as well. Two things that will exacerbate this will be high cadences with low resistance (i.e spinning) and low cadence with high resistance (i.e. grinding at like 40-50rpm); at least when you first start cycling. Pedaling drills help adjusting to these settings (at least they did for me). Zwift has some pedaling drill work-outs that might help. Just rest and rehabilitate the injury and it should improve. If it does not, or if you have sensations of instability in your knee, you definitely need to see an orthopedist. Specifically a sports medicine orthopedist. Again, this is not me giving you medical advice or a diagnosis, only speaking in general.