Can I take Plan B while spotting?

Yes if needed. If you are within 72 hours of unprotected intercourse and are not on birth control then taking Plan B can decrease the risk of getting pregnant. Spotting now will have no effect.
Yes. Plan B can be taken at any time during a woman's menstrual cycle. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.