Hi - if i were to guess - and i am only guessing - you were sitting on the hill with your foot tucked under a bit - like pointing your toes and pointed in - corss legged sitting, sitting in "mermaid pos" or your feet tucked to one side etc. If not, then i am not sure but twisting your foot like that for a while can cause "creep" in the tissues which stretches them too far and then you can get issues.
You should really be seeing someone about it. Like we said - it would only be once or twice. What is the price of 2 sessions of physio compared to missing out on regionals?