So first, don't put too much faith into any rumors about her promiscuity. Most often, rumors aren't based entirely in fact.
In any case, you had no risk if none of your genital skin touched hers. STDs don't go through fabric, no matter how thin it is.
There are no STDs that would cause symptoms on her thigh/knee like you describe. I don't know what she had there, but it wasn't an STD.
You don't have to keep your shirt on during sex to avoid STDs. The only way STDs are transmitted is mouth to genital contact (oral sex), mouth to anal, genital to genital contact (rubbing, grinding, penetrative intercourse) and genital to anal (rubbing, grinding, anal sex).
Your chest touching her chest won't transmit an STD.
It's also okay to talk to your partners about STD histories and testing. You don't necessarily get to talk about who they've been with or how many, as that isn't your business, but their STD status is. This is a good time to bring yours up, too. It's a conversation, not a confrontation, which is good to remember. This is a topic that goes both ways. :)
For this encounter, you had no risk and don't need to test or worry.