Hi, Thanks for using HCM.
It could be due to ligament tear or stretch in tendon resulting in pain and swelling.
Need to get examined for it, any tear in ligament needs correction, if not corrected more prone to get torn with minimal work in future.
Need to get MRI
of knee for its detection.
Any exercise needs to be started slowly and gradually you can increase level of work. your body needs to get adjusted for it.
Hope I answered your question. Consult your doctor for further examination and management.
Feel free to ask me if you have any further queries.
Wish you good health. Take care.