What OTC product will relieve my RLS & why do antihistamines cause it to flare up?

Hard to say. There are herbal remedies for rls, including valerian root and others, but they have not been proven to be either safe or effective. Avoidance of caffeine and nicotine may help, as may massage, relaxation technques and acupuncture. Some people with RLS are deficient in iron and folic acid, and supplements of these may be helpful if that is the case. Nobody knows the cause or why things bring it on.
Restless Leg.... ..Syndrome (rls) is almost always aggravated by antihistamines, so you need to avoid them entirely. Treatment involves ruling out underlying problems, such as iron-deficiency anemia. Eliminating tobacco, alcohol, and caffeine cures many people. Get enough sleep and exercise regularly. If none of those things work, you'll need to see a doctor.