I am not expert at all about piggies and such issues, but what I know is that guinea pigs are used as cavies because their hormonal response is just the same we humans have. Hormones influence the brain and behaviour and this is a well known fact, although the subject is still half misterious. Maybe (maybe!) a piggie (who is a social animal) can develop some weird attitude as much as we also do sometimes in several periods of our life. What to do? calling a psychologist? prescribing antideprassant or other similar stuff? of course it is not possible and it would not even be right...
I would not do anything...
I remember when I was a child and my mother used to invent any trick for introducing me any new friend, even when we were at the campingsite and I was so happy playing with the sand on my own or reading comics or even becoming fond of ONE girl (or 2, not more), usually always the german girls (don't know why... ).
Of course it seems a different story, but when I see my daughter, my piggies and even my husband on their own I simply don't care and consider this as a natural difference. And as you see, the reaction of a sow forced to live against her nature is always very... natural. I also would become aggressive if I was forced to live in some innatural situation (although of course we have an intelligence which contains our bad emotions ).
I would let her live like that, cuddling her even more; maybe she wants only a human mate now; and maybe she will change again...
My two sows were considered inseparable at the rescue and they have lived close each other for some time. Now they don't fight, but they want to be on their own, each one into a hidey where the other one has no access, although the hidey is very large and some months ago they used to sleep together there. During floor time they take a rest in two different corners of the kitchen. When one is in season the other one must keep her distance and if none is in season there is silence and peace but they don't sleep together.
My former piggie lived alone popcorning and showing happiness to us (more than these two sows). Your sow is healthy, maybe something is moving into her brain, something not known to the vets, but I would let her live as she prefers...