What is Mayella claiming happened to her?

Mayella claimed that when Tom came into the shack, he raped her.

Mayella claimed that she asked Tom to bust up a chifferobe for her for a nickel. When Tom came in he forced himself on her. Her story, of course, has more holes in it than Swiss cheese. Mayella is so conflicted, angry and confused that she blurts out one of the most poignant speeches in the book,

"I got somethin' to say an' then I ain't gonna say no more. That nigger yonder took advantage of me an' if you fine fancy gentlemen don't wanta do nothin' about it then you're all yellow stinkin' cowards, stinkin' cowards, the lot of you. Your fancy airs don't come to nothin' – your ma'amin' and Miss Mayellerin' don't come to nothin', Mr. Finch-" Then she burst into real tears.(18)

Clearly this girl is harbouring some pretty hideous secrets and they have nothing to do with Tom Robinson!

that tom robinson had just came to her room and raped her while she went for the nickel to pay him

Mayella claims that she told Tom Robinson to bust up her chifferobe, but then he burst into a shack and forced himself on her. "I got somethin' to say an' then I ain't gonna say no more. That nigger yonder took advantage of me an' if you fine fancy gentlemen don't wanta do nothin' about it then you're all yellow stinkin' cowards, stinkin' cowards, the lot of you. She refuses to say anymore because she clearly is fabricating a story.

On the other hand when Tom Robinson is testified, he claims that while he was busting the chifferobe, "she'd grabbed me round the legs, grabbed me round th' legs, Mr. Finch" (193). "She says, kiss me back, nigger' (194). This provides us with another possibility of Mayella being guilty and not the innocent victim.