I'm on midnight watch with our 6 week old daughter as I'm typing this. Your girlfriend just created a whole 'nother human being, then either forced it out an oriface or had major abdominal surgery, and is now expending about 1500 calories per day making milk to sustain the little being. While this is happening, her brain and body is swimming in a hormone soup to form an immense bond with baby and begin resetting her body. She's exhausted. She feels gross and sex is the last thing on her mind. Doctors recommend no earlier than 2 months for vaginal birth and 3 for c-sections.
Like it or not, when you signed up to become a parent, you gave away the "lead character" role. Both of you exist to keep the newborn thriving right now. You will have a chance to be intimate with your girlfriend again. But it will be on your child's schedule, not yours.