Hi Jlee,
What you describe about your POIS sounds very typical symptoms ( fatigue, speech problems, ...), and in your case, it seems a mild to moderate case of POIS. The more severe POIS symptoms you describe when having more than one release in a short time span is also a common feature for POIS sufferers.
However, muscle aches are usual, but your specific muscle strain problem is not. I cannot answer your question, you will have to find out for yourself - all I can do is share some hypothesis. In my view, what's happening at the moment of ejaculation has influence on the type and severity of the POIS symptoms coming after ( for example, in my case, the more I let go, the less symptoms I have, and if I ejaculate while trying to hold it back, POIS will be more severe). So, I guess that in your case, the state that a group of muscles is in at the exact moment you ejaculate may have an influence of what is happening after. I hear you that this strains do not follow the same resolution pattern as other symptoms does, but it may be a "hybrid" symptom, partly due to over-contraction and partly due to POIS. So, if at the time of release, some muscle are strongly contracted, and have been for a certain time, it may cause a specific happening there that do not happen in the other, relaxed muscles. You could do some tests yourself, to verify that. Like, masturbating in this "hard core" mode for a certain duration, but not going through, and not ejaculating, and see if you get muscle strains. But of course, I am not at ease with this idea of very muscular and harsh masturbation, so my best advice would be the following:
Obviously, the easy answer to your strain problem is simply to stay relax while you have an O, and stick with the "soft" mode that does not cause you these problems. Anyway, being to harsh and muscular during sex is usually not the way to go, in my opinion. Some guys go so far in this way that they have a hard time enjoying normal sex with a normal woman, they find this too "muted". I suggest you take simply a more natural approach to this, and bring down a bit the "intensity" knob.
I hope this will help you clarify all this and find your own answer to this situation.