Small soft spot on ceiling below shower

Appreciate some help here before i spend money in calling an expensive plumber because i think the problem can just be poor caulking in my shower. I noticed a small soft spot on my ceiling today. I pressed it and could sense the softness, except for that small spot all areas around it was solid. I dont see any staining in the area. Hence i think there maybe a small drip from my shower above (because of poor caulking) in that area. How can I confirm this. One way is to caulk and just see if the spot becomes worse but that may take a long time. Is there anything else I can try? Any suggestions

Please see pic below. The bulb and trim is in the picture to give an idea on the size of the soft spot