After some testing, I determined that there was some dirt on my 18-50mm lens.
Here is the photo of a white wall with my 18-55mm:
You can see the settings of above photo here:
Here is the same wall with a 27mm lens:
It is 'spotless'.
So I determined that the dirt must rest on the rear end of the 18-55mm lens.
However after cleaning my lens, the situation got worse. Not only did one of the spot remain, it became more prominent; there is also a string of fiber adhered to the surface.
I used the following to clean my lens.
How can I recover from this?