If you live in an area that sees a lot of fluctuation in temperature as the seasons change it is likely that you will experience a change in how your ball fits your hand. In colder weather our fingers get a bit thinner and when it is warmer they expand. I luckily do not experience much change in my finger tips, but I do see and feel a change in my thumb.
A few weeks ago the Pittsburgh area finally realized that it was autumn and that it had some catching up to, dropping from days in the 70s to days in the 40s and nights in the 30s. With that drastic temperature change came a quick change in my thumb size. It also just so happened that this change occurred when I had to use a ball I don't often throw and so I was a little slow to pick up on what was happening. The ball was coming off my hand early and so I subconsciously began to squeeze it during my follow through, which resulted in this:
That is a blister that developed from the friction of squeezing the ball for three games. It stings, but doesn't hurt, and usually heals up enough in 7 days that some New Skin can be applied pain-free. In about 10 days it is completely gone. If you ever develop something like this check your fit because you are most likely squeezing the ball.
An easy fix here is to apply a piece or two of tape in your thumb hole. Always be sure to place the tape in the back of the hole instead of the front to avoid altering the span. Another quick trick if you don't have any tape is to blow into the hole before inserting your thumb. The little bit of moisture provided by blowing into the ball will cause your thumb to grip the inside of the hole without you having to squeeze the ball.
Off the record, or as off the record as I can be on a blog, alcohol consumption could help as well. As you may have noticed, drinking alcohol gives you a warm fuzzy feeling because it dilates the capillaries and brings more blood to your skin. This also has the effect, along with the added liquid, of swelling your fingers and thumb. Short of having your ball plugged and redrilled every time you need to change your fit, though, this is one of the more expensive options available.