could be that you have not opened up the microcosmic orbit sufficiently. I feel that Master Chia's ambition to benefit beings by exposing the secrets of sacred lovemaking skills may have led him to under-emphasize some of the traditional aspects of training in the recent books. As i was taught, classically, one should open up the entire microcosmic circulation over a number of months in order to have a free flow of sexual energy through this channel. Otherwise, when one tries to pull this thicker energy through, it can run into blockages and cause pain and disfunction.

Another possibilty is that you are not pulling up completely and so are "leaking" some energy this is usually still better for health than full ejaculation, but if you are having sex frequently, it can add up. If you have any sort of underlying deficiency of the Kidney energy according to chinese medicine - this subtle loss of qi will over time result in back pain and weakness.

Definitely agree with alchemy to try the TCM modalities, and work more with opening up the microcosmic circulation to get that qi transformed. Your best bet would probably be to find a Qigong healer or master who can treat you as well as give good advice on your sexual practice.

good luck.