A year and a half ago "a friend" invested money in my small business to help with a move.
She recently decided that I was too difficult to deal with and that she no longer wanted to be part of it. She is now calling that money a loan and demanding payment.
There was no partnership agreement, no papers signed, no agreement regarding payback of money invested.

She is also sharing information with others regarding the amount she says that I "borrowed."
In fact I dispute the amount.

What are my rights? Would what she is telling others be called slander?

Anyone can sue anybody for anything, just about. Whether they win or not is another matter.
Small Claims courts hear countless 'loan' cases between friends and lovers, and there's no easy answer to predict who will win or lose. The judges have plenty of discretion and use it to keep things moving quickly. A lot depends on who is more organized, concise, and respectful.

Slander is very difficult to prove to you have been harmed and what the truth is anyway. You can't have it both ways - you say no loan, no written agreement, no clear amount, so how is a court going to decide anything she says is slander, never mind how you are defamed?