A:  The fact that your girlfriend is on the pill puts the odds in your favor. Ejaculating inside of her instead of pulling out won't raise her risk of pregnancy much at all.  Essentially, you have been using two birth control methods up to this point: 1) pulling out (which can be up to 95% effective in preventing pregnancy if you're good at self control in sexual situations), and 2) the birth control pill (which is generally 98-99.5% effective when taken correctly and consistently).

As an aside, while your current pregnancy risk is very low, a drawback to both of the methods you're using is that neither offers any protection against sexually transmitted diseases (STDs).  If you have any skin to skin contact or penetration, spreading an STD is a real risk.  There are many viral diseases like HPV (genital warts), HSV (Herpes Simplex Virus) or Molluscum contagiosum.  And, of course, HIV/AIDS is a risk.  Bacterial diseases include gonorrhea and chlamydia.  Parasitic infections include crabs and scabies.