I think I rubbed into a patch of posin ivy; is there preventive medicine I can take to block any rash?

Poison ivy block. There is a topical you can put on called ivy block, that can help to stop the plant resin from direct contact to your skin. However, best advice is protective clothing. You can add ivy block cream to protective clothing, but not in place of it.
Poison ivy preventio. Recognise poison ivy and avoid it, wear long sleeves, long pants and gloves if you suspect poison ivy, get rid of it if you recognise it if you get exposed to it, immediately remove all your clothes and put them in washer and take a quick bath, to wash of the ivy extract asap, if you see rash put cortisone crream and take Benadryl (diphenhydramine) to stop the itch. If the rash still gets worse see the doctor and you wil.
Barrier cream. Barrier cream containing bentoquatam (ivy block) can help. Also wash the area exposed to poison ivy with water and soap for 5-10 min right away.