It's what tells your body to build calluses. Play till you can feel a blister start to build under your skin. Stop for the day.
Play again the next day, not to that point, but let it hurt.
After 4 or 5 days of it, you will see the skin thicken and the pain will start going away from there on.
Fresh calluses will go a way fast. 3 days off will erase 5 days of pain. So stay at it even if you just press your fingers on the strings as if your playing while you watch tv or what ever.
Don't play till the blister pops, That is an injury that has to heal. But they seems to build faster if you play till you can feel the blister.
It hurts, but in the long run....... it works.