I got cramp in my calf while I was asleep and it was really painful.
My calf cramped up while I was asleep at night and it really hurt.
While I was asleep at night I got cramp in my calf and it was extremely painful.
ふくらはぎ(など)のけいれんは「cramp」と言います。「cramp」は動詞として使う事が出来ますし、名詞として使う事も出来ます。
「夜に寝ている時、足がけいれんしてとても痛かった。」= I got cramp in my calf while I was asleep and it was really painful. / My calf cramped up while I was asleep at night and it really hurt. / While I was asleep at night I got cramp in my calf and it was extremely painful.
ボキャブラリー
calf = ふくらはぎ
while = ~の間、~の時
asleep = 寝ている
really = とても、本当に
painful = 痛い
hurt = 痛む
extremely = 凄く、非常に