I'm 19 weeks pregnant and used selsun blue (active ingredient: prythione zinc) without thinking. Is this OK??

That's fine! Topical zinc is safe in pregnancy. From my database: "Based on experimental animal studies and poor dermal absorption, used of zinc pyrithione shampoos is not expected to increase the risk of congenital anomalies"
SELSUN PREGNANCY. ACCORDING TO SOME REPORTS, SELSUN SHOULD NOT BE USED IN THE FIRST TRIMESTER OF PREGNANCY. BUT DO ASK YOUR OBSTETRICIAN . . .