Cramp

( I had a leg cramp while I was sleeping last night)

I have a cramp with my leg while sleeping once in a while.

( I get them once in a while )

I know how to fix it but it was strong one last night so I had pain for a couple of times.

( I know how to fix them, but they were strong last night and returned a couple of times)

Finally I could relieve my pain and I went to bed again.

( They finally relented and I could sleep again )

Cramp

痛いです！

Additional notes:

Line 2: "I have a cramp with my leg while sleeping once in a while." This line repeats a lot of the words from the first line. If you don't want to repeat, you could say this a couple other ways:

"This happens to me once in a while." (This is good, but it's not very specific)

"I get leg cramps at night from time to time." (This says the same things as your sentence, but using different words)

Line 3: "I know how to fix it but it was strong one last night so I had pain for a couple of times." 

Below are two other ways you could say this. I used different colors to show how parts of the sentence can be moved around and changed.

"I know how to fix it, but last night it was a particularly bad one, and the pain came back several times."

"I know how to fix it, but it was really bad last night, and it happened a couple of times."

Cramp

Glad it went away and you could sleep!