Kinda urgent! How do I know I'm not pregnant???

Hello, I've been an avid worrier for a year or so now, so I understand your panic. Chances are if you were protected, you'd be fine either way. Implantation bleeding usually happens about two weeks afterwards so what you experienced was probably your period being early OR your body just adjusting. Sex is known to cause a little bit of bleeding for some people the first few times.
Even if the world is telling me everything is fine and I'm not pregnant, I always take a test if I feel uneasy (which is most of the time). If you can't buy a test safely, check for local clinics. They will not judge you. My local clinic offers support for people 13+ with no judgement, so if you can find something like that then it is definitely worth visiting, even if it's just to ask what kind of support they can offer. If you're in the UK they can fit your chosen type of contraceptive if you would like to feel more safe.
I would always advise talking to an adult that you know and feel safe with but if that isn't an option then clinics are such a safe place. Just be safe and always prioritise feeling safe with the person you're with!

Thank you so much! I really needed to just hear it from another person that the chances are really as slim as I thought they were
I live in India, where patient privacy isn't really something that applies to families. So if I do go into any clinic for any kind of procedure or test, even though I'm 18, they will inform my parents or call them for consent. There's no way to avoid it... (Actually, for this exact reason that young people go get things done out of fear and end up doing something probably stupid)
Even though everything can be bought over the counter, and I actually can just take a test, I'm really scared of what I might see in it...
I know I'll end up waiting until I get my next period to see what happens, and I'm going to be worrying the whole time, but you really helped put my mind at ease a bit, thank you

I'm so sorry to hear that patient privacy isn't something you can access, but I'm so happy I've helped you a little bit.
I completely understand the worry about seeing something you don't want on a test, you're not alone on that. If you would like to be a bit more confident with taking tests, I might suggest that you take one right after a period so that you can experience what it's like to buy and take one, without the panic about being pregnant (since, yknow.. you'll have had your period). It might seem like a waste of money but it may help if you do need one in future.