I had a Mirena IUD and after sex the only foul discharge smell I had was from semen. Maybe I just dont like the smell of it but it was gross to me. It was never green though and after a shower the nasty smell was gone. I would check with your doctor about an infection.
Ps. I have heard that douche isnt good for you. It throws off a womens natural PH balance. I am 34 years old and have never done it. I have only had two pregnancy related yeast infections my entire life. Avoid the douche and talk to your doctor
Green discharge is never a good sign. You need to see an ob/gyn and get tested for an infection. It could have nothing to do with the IUD, but if you do get an infection that turns into pelvic inflammatory disease, having an IUD in might make things worse (and PID can cause sterility all by itself).