If an impurity was slight and was pardoned, but then it spread and became no longer slight, does the pardon for it remain, or must it be purified?

It is evident that you are experiencing some waswas (whispers/obsessive thoughts), and for this reason, we advise you to disregard them and pay no attention to them, for that is the most beneficial cure for them. Regarding a small amount of impurity that is pardoned: if it mixes with a liquid like water until it is no longer a small amount, then it is not pardoned, but rather it must be washed. If the impurity is less than the area of a dirham (an ancient coin, here meaning a small, specific size), then it is pardoned, even if its quantity is more than that of a dirham.