I do not have any STI. This hard yellow bump under the foreskin has been there for years. I am unable to retract my foreskin, hence I am unable to check what it is. 
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There are several things this could be: as you are not able to retract your foreskin this could be an indication of the build up of smegma between your inner-foreskin and the head / glans of your penis.
Or, it simply could be something that has developed this way - on any of the layers of skin in the area, or perhaps on the glans itself.
It would be best to expose the head of your penis so that you can keep the area hygienically clean and prevent issues caused by the build-up of smegma on the head of your penis.
What age are you now?
Have you even thought of stretching your foreskin so that you can begin to expose the head of your penis?
Is circumcision something you would consider, or would you prefer to avoid this?