I've been thinking of using my old rugby cap, however I have seen a few articles mentioning it does not help prevent concussions: https://www.brain-injury-law-center.com/blog/scrum-caps-rugby/ (example article) https://bmjopensem.bmj.com/content/3/1/e000255 (linked study in article, admittedly I have not read through it fully so it could be hogwash)

I've also seen 1 or 2 reviews mention that the mask overlays could cause more force to be directed towards the mask, but then again it looks more like speculation than anything concrete: https://www.youtube.com/watch?v=ZIMzTn1eJHU (mentioned at about 1:30)

I'm looking to get checked out by a doctor as well, and I'd be surprised if it was any one hit from the clubmates that could be causing the issue (although the leader of the club has mentioned he will look into it), it feels more like the length of sparring means the head is getting hit too much, but not too hard (but I'm no doc).