Lead bullets mean lead poisoning in the blood stream. Especially after being out of commission for a great portion of the arc, being carried around, he probably let the lead sit in for quite a while. So, after he was exhausted from carrying him and luffy into the palace, he coughed up blood and found the traces of lead in his system. He's the only ranged fighter in the duo, and he couldn't do much because it was right after he extenuated himself, and Luffy was aware that he couldn't touch her, so he couldn't do anything either.
But, to be honest, while there was a lot of talking, Usopp's actions were pretty quick, so it'ws not like they were hours standing around for Sugar to do her business to them.
But what I came here to say was:
HAAAAAAAAAAAAAAAAAAAAAAAAAAAAAKIIIIIIIIIIIIIIIIIIIIIIIIIII! Usopp has HAKI! OmG GODSOPP!!!