Yes, phenergan could be responsible for the increase in RLS symptoms that you are describing, because it is a medication that functions by interacting with the nerves in the gut, but does have crossover that impacts nerves in all areas of the body to some degree. Even more important for RLS, it blocks dopamine to some degree in the brain, which is one of the most involved molecules thought to be important in the development and presentation of RLS symptoms.
The first step, which you have taken already, is to avoid the medication when it is safe to do so from the standpoint of your other medical condition. Speaking with both your primary care doctor
as well as the person who prescribed the phenergan is likely to provide additional information, such as alternative therapies. You will also need to discuss your symptoms with your provider and determine if they are significant enough to warrant further therapy with medications designed to address the RLS specifically. Fortunately, there is hope that by ceasing the phenergan you will soon return to your baseline of symptom free life with regards to your RLS. Please speak to your physician as needed for this concern.