Don't take any antibiotics before a transplant without talking to your doctor first. I'm not an expert - not even close - in transplants, but the odds of you getting anything from giving oral sex are very low, and not worth the risk of messing up the transplant.
I'm assuming you gave oral sex? Giving oral sex to a penis puts you at risk for oral gonorrhea, syphilis and possibly chlamydia. The risks of all of them are low. Syphilis isn't that common, nor is oral gonorrhea, and oral chlamydia is even less common.
It's not something I'd worry about at all. Talk to your doctor, and I imagine they might give you some antibiotics after the donation to prevent infection, depending on what you are donating. Depending on what those are, they may prevent/cure STD infection. Ask your doctor about it.
And congrats on being a donor. You're a special person. :)