) When you stand put your weight through your other leg. If it is a slight tendon strain I don't know if there is anything you can do but rest. See the doc, or use one of those NHS drop in centres, to make sure you don't do any damage without realising it.
You could give NHS Direct a call on 0845 4647 or visit [url]http://www.nhsdirect.nhs.uk/[/url]. I've not used the website but have found the phone line very helpful. They take your details and call you back when they're free. Be warned it can be a while later, but they do always call you back. Don't know how they fair on sports injuries.