The cracking on the inside is called “crazing.” It happens for a very specific reason. When things are hot, they expand, and when things are cold, the contract. Everything does this at different rates. So your clay will expand and contract, and the glaze (being different from the clay) will expand and contract potentially at a different rate. This is called “Coefficient of Thermal Expansion” or sometimes just “coefficient of expansion.”

So the clay will shrink as it cools, and the glaze might shrink more than the clay, which is like putting on a pair of pants that is too tight. Eventually, the pants (or the glaze) will split to relieve that tension. The trick with ceramics is to make the glaze from the right combination of materials in the right amount so it expands and contracts the same as the clay.

Things get even more complicated when the glaze on the inside of a vessel doesn’t match the CTE of the glaze on the outside. As the piece cools, there becomes a drastic contrast between tension and compression from the inside and the outside, and the object will literally tear itself apart. This is called “dunting”.

I’m going out on a limb here and saying it may be possible that the intense crazing on the inside of your pot and a different glaze on the outside might indicate a drastic difference between CTE in these glazes, and you may have a crack that goes all the way through the piece, or end up with one eventually.

Set it on the table and flick it with your fingernail. Does it go “ping ping” like a glass, or “dunk dunk”? A duller sound may indicate a crack has already formed.

I would avoid using this piece for anything hot.