That is true but you still did the right thing. Because depending on how long she has had the possible std, it may be present enough in her vagina to pass it on to you with one prolonged sexual encounter. (Say a half hour or longer). You just never know. But there are other things that can cause the burning like a yeast infection. That is not an std. So, don't think the worst.
I hope she gets tested too so if it is an std, you both will be treated and stop it from spreading to other people.