I am writing an essay and I have previously written and submitted an essay to a different professor about a similar subject. Would it be considered plagiarism if I copied my own exact sentences from my previous essay?
Yes, it is called self-plagiarism. This happens when you reuse significant portions of an earlier work without acknowledging this. This can be a major concern if the prior work is copyrighted as it infringes on the rights of the publisher.
If the work comes from any other sources, it is wisest to cite where it came from, even if it came from you.