Hi
In answer to your question there is a 2 main reasons why you have the pain.
The first is that semen (cum) has a high salt and zinc content in the protective fluid that surrounds the sperm. This will sting and irritate any wound.
The second is that the penis is actually controlled by muscles that pulse at the base when you ejaculate this forces the eye of the penis to stretch.
The movement again will cause irritation to the wound as it is similar to pulling on the ring.
It will take about another 4 weeks or so for the healing to finish.
Take Care
Matthew