agree with posts so far. sounds like mild musculotendinous junction tear of gastroc or soleus. should settle as discussed. unlikely due to your description of the mechanism of injury - but if it doesnt feel improving at all within 2 weeks, you need to be aware of partial rupture and retraction of some of the muscle, relatively serious dependent on extent. as no warm up, you could also have possible nerve injury, overstretched component too. again not necessarily serious but can affect the healing time. always good idea to get physio etc to check it out for you to give some advice over extensiveness of injury and likely recovery time as very hard to diagnose accurately over internet.