Because every item on Amazon can have many different sellers, some of them have the same product in the same Amazon warehouses. OP added the item to their cart using the default seller, it just so happens that the seller also raised their prices that day. So the price went up in OP’s cart.
Searching the product on Amazons store likely still said $30 because Amazon switched the default seller to the new cheapest one, which was no longer the seller that OP added to the cart.