Notice small rush in my private part. and not sexual active for three month. two things that changed from my side. i sweat there and take multivitamin?

Question is unclear: but you seem to have a rash. Most common where there is sweat would be a yeast infection. Ask pharmacist for over the counter remedy. wear loose underwear. Mulivitamin use is irrelevant.

Sweat : Probably caused by sweat. Try OTC Hydrocortisone cream daily.

Where on : Your private part. It could a fungal infection but it needs evaluation and diagnosis. Vitamin will not change it