Hmm. Are you doing the kick with a slight bend in the knees? I think that's what most people do for the side stoke, as the path (but not angle) of the feet more closely resembles the breaststroke kick than freestyle.
I'd let your knees rest for a bit before trying it again, so as to not aggravate the injury. Try it with again making sure that your knees do not become locked at any part of the kick (assuming that you might be doing that now).
Try doing the kick slowly, with a kickboard to support your arms. See what happens as you slowly increase the intensity of the kick. That's how I'd try to diagnose the problem. If it's still happening, I'd go see a doctor, trainer, or physical therapist.
Hope this helps.
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