Is the foot more mobile now? It might be some random cramp thingy, or maybe you pulled the tendon a little when you stood up. Try soaking the foot in warm water and wriggle it about a bit, and keep the ankle warm and cozy for a while.
If you have sprained your ankle then I guess you've been walking very gently on that foot for the last days and the Achilles tendon might have stiffened up from lack of use, hence the cramp/pulling now.
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