I have the same or similar pain deep in the canal some hours after ejaculation. And just like you, Zenman, I think too that I may have brought this condition on my own by practicing Tantric techniques to achieve multiple ogasms by stopping stroking right before ejaculating, then start again, over and over and over. The ultrasound shows that canal is widened in that area that hurts and so its possible that it's caused by hardening leftover seemen in the canal.
Please let me know whether your condition improved and how.