Hello, I'm wondering... I've been having protected sex all my life and I know my partners statuses... But recently, I slept with someone only to find out that she sleeps around (she's been with a lot of guys) and I'm just really paranoid to think that I have some STD or something? I'm not feeling any different but HIV only comes up in like 3 months? So my main worry is AIDS... I Asked her about it, obviously in a nice way, and she said that I don't need to worry, basically implying that she doesn't have... Opinion? Do you think I should go for the test in 3 months or not? How does it work? How much is it? And what will happen if I go there?

Anyone who has unprotected sex with someone they don't know really well can contract all sorts of STDs, so sure. Call a local walk-in clinic and ask all the questions you want. You save money at a clinic by seeing a nurse, and some are sliding scale. We can't know what it will cost at any given place.

Just because it has higher odds and is untreatable, I'd be more concerned with herpes.

Even if you "think" you know the history of your sexual partners, guess what, even they may wonder sometimes.

So it sounds like multiple partners (at least the way read it) so yes you should be getting regular checks every 6 months or yearly anyway.
My work requires one every year.

But yes you should, with this case