Hello Alli, Adeniums are such sun loving plants that loves warm to hot temps, and typically depending on location and temps, it may just be waking up now or already awake. Mine is just barely waking up this year. It has been one quirky cool Spring season. They typically go dormant during the cold season and will then wake up this Spring and do their lovely active growth during Summer.
Looking at your plant, I am a little thrown off track since the stem is quite slim, maybe it is not grown from seed? You did right by cutting off the rotten part. Let that top half callus an dry. I would also inspect the roots of the remaining part just to see how the roots below soil line looks like. Typically, if the part under the soil is looking too soft, your media is too wet, so need to unpot and get that lower part to air dry, otherwise it will just continue to rot.
They are succulents since they hold water in their caudex (stem to root part), so it is really important to have a very well draining media and wait for new growing leaves to be sure it is already properly awake from its slumber.
It is also possible it is not an Adenium, so still I would unpot and check the main plant. Got to see condition of the roots.
