If a professor already graded my work can she come back a month later and say it was plagarized? ?

Charges of academic dishonesty are handled outside of normal grading channels.  They are usually submitted to a separate committee and a hearing is held.  Actually, I would appeal and demand a full academic dishonesty hearing.  What she was trying to do was to justify the poor grade she gave you by remaining silent about what she thought was your previous plagiarism.  If you REALLY did not plagiarize anything, here is what to do.  Send her 2 letters by certified mail c/o her department, one to her, and one to the chair of the department. State that you are confused and insulted by her accusation of plagiarism and you DEMAND a full disciplinary hearing by the appropriate committee.  In the letter to the department chair, tell him or her that you are sending the letter to them to ensure your rights and inform him that one of his staff is behaving not in the best interests of Academia by using grades to settle a personal score or prejudice that she has against you.  Also include a copy of the letter that you sent to the prof. Of course, if you WERE plagiarizing, this course of action will thoroughly f**k you, so be damned certain you know where you stand. good luck

You are at university and you write like that?   You should not revert to almost 'baby talk' and lack of punctuation, and lack of revision of your text just because you are asking a question on the internet.

Maybe you and your instructor have a different opinion of what plagiarism is? You think you're 'researching" something, she thinks you're stealing? Ask her to give you a direct example of what she believes to have been plagiarized. I think one of you two, most likely you, will have an ah-ha moment. Good luck!

Sometimes plagiarism allegations - and proof - crop up after the work was submitted & graded. Perhaps she investigated more thoroughly? Did you plagiarize anything at any time? One incident of plagiarism puts ALL your work under suspicion.

Yes, plagiarism can be brought up any time, and a grade may be changed for academic dishonesty even years are the class or degree was completed.

If you are completely sure you never plagiarized anything, then call her bluff and appeal the grade.

If you did plagiarize, getting a lower-than-expected grade is probably the very best outcome you could hope for. You should consider yourself fortunate that you weren't expelled.

If you're not entirely sure what the definition of plagiarism is, then look up your university's academic honesty policy on their website. Even paraphrasing your class textbook can be considered plagiarism, if you didn't cite it properly.  

Yes.  If  plagiarism is detected after a grade has been submited, that grade can absolutely be changed.  That applies not just to assignment grades, but even to entire course grades.  If your professor is just dropping your grades because of plagiarism then you should consider yourself quite lucky.  It's at least an automatic 0 on plagiarized assignments in many cases, and it's not unncommon to automatically fail the course and be turned in to Academic Affairs (or the like).  

If you truly didn't plagiarize, then you can and should appeal.  But if you did, even just a little bit, then I wouldn't push it.  Things will be much worse for you in that case. 

The answer to your question is yes.  Colleges have no statute of limitations on academic dishonesty.  That is why the alma mater of a certain well-known politician is currently being pressured to investigate allegations the politician paid someone else to take his SAT decades ago.  If you did not cheat, you should appeal the grade.  If you did cheat, consider yourself lucky not to be dismissed.

She discovered later that you had plagiarized.  Happens all the time that the paper seems okay then realized that the system flagged the fact that you wrote none of it.  Do your own work in the first place and you have no problems.  If you truly did not plagiarize, then appeal, but know that if you did and claim you didn't and the teacher proves you plagiarized, most universities will simply have you academically disqualified.  Pick your battle carefully.  If you DID plagiarize you are getting off lightly.

Meeting up with inconsiderate, aggressive, bullying teachers is one of the facts of life that a student learns about.  The only hope is to appeal to the principal, but very few people ever would do that out of natural fear that young people have.