My girlfriend is six weeks pregnant this Monday coming which matches or dates is was with her, but she also had sex five days later with someone else but took the morning after pill, is there a chance the baby is not mine?

Yes there's a chance. Due dates are just a best guess, even ultrasounds cannot determine the exact date of conception or the date the baby is due, especially since there's only 5 days between the time she had sex with you, and someone else.

The only way to determine if you're the father is a paternity test (DNA test) after the baby is born. Everything else, ultrasounds etc, are a best guess, not an exact answer. So wait until the baby is born and get a DNA test done to establish paternity.

Good luck.