You didn’t say what your hygiene is like and whether you had retractable or non-retractable foreskin or if you had foreskin at all.
For example, if the tip of your penis has been constantly under the foreskin and you have just started retracting, the tip could get sensitive because of that since it has always been covered.
If you haven’t been cleaning regularly under the foreskin, the white stuff (called smegma) could have accumulated and caused an infection or inflammation.
You could have balanitis which is an overgrowth of organisms that normally live on the skin in this area. Balanitis may occur because of poor hygiene or sexual acts sometimes but it is not sexually transmitted disease.
Balanitis is rarely treated with medications and maintaining good hygiene should sort out the problem.
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It is not meant to substitute for medical advice provided by your physician or other medical professional.
If you have or suspect that you have a medical problem, promptly contact your physician or health care provider.