Why do I still have mouth swelling and throbbing pain 3 weeks after implants?

Three weeks after two dental implants the inside of my gums are still swollen, painful and throbbing. The dentist has x-rayed them and all is fine, no infection, no movement, but I would not expect pain at this stage, as everything I've read says it should be better by 10 days. I have inflammation in a sinus near the site and I have autoimmune thyroid disease which I understand can lead to soft tissue problems. At what point should I be thinking this has failed?