Well even though you say that you're fairly fit you may well be exercising different types of muscles... I'm fairly fit but last night I did a circuit class for the first time and you'd think I'd run the marathan after I came out as I was exercising a different way to what I'm used to. Also, really can't emphasise decent footwear enough. Perhaps it's a combination of these things as well as maybe needing to warm up for slightly longer (at least ten mins) that have resulted in injury, it really is just so easy to do.
Rest up, keep elovated and cold/hot compress to reduce swelling. Ibu - profens will treat any inflamation if you have any and if it's still bad then quacks on MOnday. No point in exercising before it's properly healed, I also did that a few years back and couldn't use the gym for months rather than weeks.
Hope you feel better soon.