I'm applying for a PhD program and I need to submit some reference letters. In December before the holidays I asked a professor I used to work with during my BSc to write one for me and she agreed to do it saying she'd do it over the holidays. The holidays passed and I emailed her shortly after but she didn't reply (normally she replies super fast to my emails). I gave her a call a week after I sent this email and she said that she hasn't forgotten about it and she worked on it the weekend before so I should receive it no later than the day after our phone call. This was last week on Monday already and I still haven't heard back from her. How do I politely remind her about the letter? Is it rude to email her about it? Because I already called her to remind her last week, I know she hasn't forgotten but I really would like to have the letter asap. There is no deadline for the application so I'm not pressed by a deadline but I'm quitting my job atm and I would like to sort this out as soon as possible (also because it involves moving on another continent), so for my own peace of mine I'd like to sort this out as soon as possible.

What do you guys think? Is it rude to email her again this week asking for the letter again? How do I do it in the most polite way?