It's relatively uncommon, but I had an issue that sounds a bit like this. After a large number of games (9-10 in a day) I had the issue of a blister developing on my thumb. This blister wasn't forming on the knuckle (where you'd normally expect it if you're squeezing the ball - I wasn't squeezing it) but on one side of the thumb, between the knuckle and the nail (about midway between them vertically).
My driller and I decided to go with a little more reverse pitch (I was already about a 1/4 I think) so we went to 3/8 and I haven't had the problem since. Fair warning, I do have a long span and long thumb so this amount reverse is right for me (I know reverse pitch is not so fashionable nowadays).
Also, to the OP: You don't indicate whether your right or left-handed, which would have a major impact on any advice. Try taking a picture of the offending thumb damage and posting a link here, so that people can be more definitive in their advice.
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Reporting from England