Why are journals telling me that my article is plagiarized even though I wrote it by myself?
Chances are the journal ran your paper through an automated plagiarism checker (the only realistic way to check for plagiarism these days) and this 70% number is what the program found. That you found nothing using Turnitin isn't enough evidence to prove there was no plagiarism: a plagiarism checker is only as good as the data it has, and it's possible Turnitin doesn't have the original paper.

Having said that the journal should tell you which paper they think you've plagiarized from, as well as which sentences are plagiarized. If they don't, you should absolutely write back to ask. From your comments you've already done that, so there's nothing to do now except wait. The fact that at least two journals have checked your paper and found plagiarism is a bad sign; on the bright side, you can ask both journals for more details and it's less likely they both don't answer.

If they respond you'll be able to fix the plagiarism if it's there, or point out why there's no plagiarism if it isn't there. 
If they don't respond after a reasonable time, then the only thing left to do is submit the paper elsewhere.