Sounds like it will be osteo arthritis unless you know you have actually done something to it. I suffer from painful joints, OA hips, knees etc. I find swimming is good exercise as it is not weightbearing. If you have sprained/strained it, then rest, apply ice, and possibly apply a tubigrip type support. See your GP if doesnt improve, may require Xray. If you feel you can exercise do a little often rather than overdoing it. Exercise bike may be okay but don't have it so you have to work hard - no resistance is perhaps best initially then adjust as you feel able.