If you had syphilis, and it wasn't treated when you took the test, you would have tested positive. It's also unlikely that a chancre would clear up within 10 days, as they usually take 3 to 6 weeks to go away (unless you've had treatment).
It can take up to 90 days to develop antibodies. It's possible that you hadn't developed antibodies yet.
Did they treat you at the time you had the sore? If they didn't treat you, test again at 90 days to be totally sure, and then you can put it to rest. Most people will test positive by 6 weeks, but a some take longer.