You may just have thready textured hair. My hair is like this and it is not unhealthy- it's just that as it grows out of the scalp, the cuticle flattens and re-curves creating the spiral S shape that is distinctive of afro textured hair.
I agree with this as well. The shape of our follicles plays a major role in how the hair is shaped and textured as it emerges from the scalp. For many of us, the texture is not consistent even along the same strand-- and this is normal.
Changes along the strand can also be a dietary thing. Shifts in your diet affect the quality of your strands coming out. If you go through a period of illness for example, the hair that grows during that period can come out looking differently than say when you were at the peak of health. Taking supplements may also affect things. You can really see this happen with your nails. They both respond to changes in diet and health over time.
And as others have mentioned, damage could also easily play a role. Sometimes flat ironing and even smoothing during relaxing can stretch the hair fiber in parts along the strand to the point where its permanently thinned (and lengthened) in one section. Most times it isn't continuous along the entire strand.