Stupid Ankle

While running last night, my left ankle started hurting. I'd only been on the treadmill for 15 minutes. I wasn't even running fast. Before I knew it I couldn't put weight on it. I can move it. There's no swelling. It's not tender. I just can't put weight on it. I don't know where this is coming from. I didn't trip, or go sideways, or get wobbly, nothing out of the norm. Stupid.