I get this a lot. There's a medical term for it, "retarded ejaculation" (basically the opposite of premature ejaculation). Hasn't been a problem if I explain the situation, and it doesn't stop me enjoying sex (if anything it helps me stay erect longer, so it's actually a bit of a disappointment if I pop and deflate).
If I see the same girl again, that tends to prove that I must have enjoyed myself, so that removes any doubts she might have had!