Hey Maxim,
Being a designer myself, I understand the situation you are in, as I've had to deal with similar cases.
This is is what I recommend you do -- in my opinion and experience, you should ALWAYS be somewhat flexible (this goes a long way trust me). Instead of getting upset about the situation, just try to handle it in a professional manner.
1) if changes are minimal, be VERY polite and just agree to the changes even though they go above and beyond what the job description stipulated and let her know that if there are any more edits to be done after this, you will need to add a milestone to the contract (if its a fixed rate contract)
2) If the changes required are going to be super time-consuming, just explain this kindly to the client (don't start a dispute without trying to communicate with your client first, this is very unprofessional), estimate the time that it'll take you and see if she will agree to adding a milestone to your contract.
Going forward, consider it a lesson learned! Next time, ALWAYS mention what the client will be getting upon hiring you/start of contract and clearly list what is in included in your proposal -- this includes the number of "edits and changes". Anything that falls outside the scope of your outline/proposal will be billed additionally and will require a new milestone for X amount.
It's very important to be clear and concise when outlining terms with your client to avoid any confusion.
