Hi lauzie
Well you know what they say, anything is possible! But see, if your cycle is 28 days and your first day of your last period was 15th May, then your ovulation window would be 26-30 May (with possible ovulation day being on the 29th). Fertile days are about 3 days b4 ovulation day, actual ov. day (where you're most fertile), and day after ovulation.
Ok now, you had sex about 10 days ago, that is on the 21st May? Sperm can survive inside your cervix up to 7-8 days but that's very rare, most common is 2-3 days I think.
Was that the only day you had unprotected sex? You sound really worried...
If your cycle is regular at 28 days and not less than that, I'd say you're safe (sorry, I don't know if you want to be pg or not).
Since you say your last period was normal then I suppose you didn't fall pg around your previous ov day (beggining of May)... If by any chance sth happened back then, you surely can test now to check.
May I ask how old are you? Can you contact a gp?
Take care
xxx