The 'soft spot' that most of us are familiar with is the fontanelle, which is the soft spot that occurs on the head of a baby. the fontanelle is caused because the bones in the skull of the baby are not totally fused yet. However, these bones do fuse together quickly after birth, and a true soft spot should not be present in someone your age.
Occasionally, if a person has a history of a skull injury
-fracture-1093">fracture or other serious head trauma, there may be a missing or displaced piece of bone that creates a similar "soft spot." However, it would be quite unlikely for you to have experienced this degree of head trauma and not already know about it.
Another possibility, is that you have a small fatty tumor or cyst under the skin of your scalp. This would likely be a benign condition, but it could be tricking you into thinking that you have a 'soft spot' in the bones of your skull themselves.
As you already mentioned, I suggest that you go to see your doctor
as soon as you can. They will review your history to make sure there is nothing concerning, like a history of serious head injury. Then, they will examine the area to figure out what might be causing this 'soft spot' and whether or not you need to do anything about it.