Help! Plsssneed to know! Could I be pregnant if sex only once?

Yes, but unlikely. You may conceive with only one act of intercourse if the timing and sperm count are right. So, yes. Fortunately, it is usually more difficult than that to conceive. If you miss your next menses or they are light, have a pregnancy test performed. Use contraception in the future. Talk to your clinician. You may also want to consider Plan B to reduce the chance of conception after unprotected sex.