If you are having symptoms that you are concerned about, you should see a dermatologist
to have your skin assessed as soon as possible. They can help you further characterize this rash and help in arriving at a diagnosis and treatment.
It is important to supplement your diet with a multivitamin, especially if you feel that your diet is not well-balanced. Multivitamins, however, are like any other medication in that they can lead to side effects and allergies. It is possible that there may be a component of the mutivitamin formulation that you are mildly allergic to that may be causing this rash. It might be prudent to try a different formulation of vitamins and see if the rash improves. Keep in mind that complete resolution of the rash could take days to weeks if it is related to the vitamins and may require additional treatment to fully resolve.
This rash could also be unrelated to the multivitamin and be a result of an allergy to lotion, soap, detergents or clothes that you have come in contact with. In addition, it could be a sign of an underlying inflammatory disease or a more serious underlying condition. You should have the rash evaluated by a trained medical professional.