I had unprotected sex nd then same nite i was put on a medicine called sulfameth/trimethoprim. i took plan b. can it cause me to get pregnant ?

No: You are probably ok, its unlikely that i day of antibiotics would affect absorption of the plan b.

Possible: Unprotected sex is how people become pregnant = there's no other way. Plan b should prevent it, but nothing is 100%. If you miss a period, use a home pregnancy test or see your ob/gyn. The antibiotic won't cause or help prevent the pregnancy.