Marda is right - most supplements say to "consult a physician" because the ingredients have not been tested for safety during pregnancy or nursing, but not because they have been proven harmful. I work in the field, and it depends on whether the FDA covers the supplement. If it's considered a drug, they do have specific requirements. For example, Flintstones vitamins and most others say to keep away from children (ironic, huh?) and say what to do if there is an overdose. But if the FDA doesn't require warnings because the item is considered a food, the FDA only gets involved if there's a problem. We see that with tainted spinach or other food-borne illnesses but they don't require warning labels - no labels on your breakfast cereal or teabags or carrots. For supplements, if there's a specific warning label about "do not take when nursing" that's one thing, but if it says "please consult" then it's a different story. Moreover, there's little risk from a one time use. Most authorities say the same about a glass of wine or walking into a smoke filled room one time with a baby - repeated exposure is one thing, but a small brief exposure is unlikely to cause anything. A vitamin has a lot less in it than alcohol or smoke too - assuming there is no specially risky ingredient in what you took. Most hair/skin/nail formulas have little of value but you can show the ingredient panel to your pharmacist.
And, if what you took was a pill (vs. a liquid or a powder you mixed in liquid), there's such a low rate of absorption anyway, it's questionable whether it would really help you at all, let alone cause a problem for the baby.
Babies are incredibly resilient.