My period is lighter than usual this month. could taking dong quai or any other supplement help?

Wouldn't think so: There are thousands of supplements advertised to do all sorts of magical remedies. Very few supplements have real research studies to show benefits (but manufacturers make claims, based on theories, animal tests, testimonials, chemical similarities, etc... but not based on real research because real research is hard to do and is expensive to do). In general, be careful about buying "supplements."