First of all, ARE you doodling on the computer or are you actually sitting and writing? This is an important distinction as much for your girlfriend's peace of mind as your own, as a writer. Everyone has days or nights when stuff just doesn't flow. But at the same time, IF you did thorough planning prior to beginning to write you should know what needs to be done each and every time you sit down to write. AND, you should do it.
What you write on those nights may be crap. Crap happens. At the same time, it is NOT our job to judge as we write. OUR ONLY JOB, in the first draft, IS TO FINISH. Ask your girlfriend (or boyfriend) to embroider that for you as a wall hanging. That IS your only job this time around; to finish this draft.
The other thing you should DO is make a commitment to writing; to WRITING TIME. Screenplays don't write themselves. So book an appointment with yourself - and take it as seriously as if it were the big ticket opportunity you hope your screenwriting may become some day. Take it seriously now, to BE taken seriously later.
And your girlfriend? Well, you might try engaging her in the process. Try READING some of your work to her; get her involved. Be careful though.... she may start canceling dates if she likes what you're writing in order to keep your nose to the grindstone. Involving loved ones in our quest can become a double-edged sword.
I hope this helps. And I do wish you luck with your screenplay. Let me know when you've completed your script. I'd love to read your work!