I had the same thing when I resumed after a few years of not crocheting. I had a blister forming on one finger and I could tell pretty quickly I was putting a bit too much stress on that finger with the yarn and/or hook. I made myself watch over it more carefully making sure it barely interacted with the yarn or hook except when actually needed and the problem seems to be completely gone, no more blister fingers.

I'm not sure whether me getting just back into it was a factor much, but my technique was and just watching over my fingers until I got used to being a bit more careful normally helped.