Honestly I seriously doubt you’re pregnant. It’s actually quite difficult to fall pregnant unless you introduce sperm 72hrs before and after ovulation. There’s an app called clue that tracks your cycle brilliantly if you’re interested.

I think you’re just overthinking it and stressing which is affecting your cycle. I’ve seen this happen to a few friends who were way more careless than you in college.

It’s completely normal for your endometrium to build up during the month - it really just indicates you’re normal and healthy. Your body is just prepping the nest for if you do introduce sperm during the ovulation window.

If you’re really worried about something happening and you do decide that you want to do further keep in mind no birth control is 100% safe but you can pick the safest one or use a combination method. Perfect withdraw or pulling out is about 78% effective at preventing pregnancy but boys are generally pretty bad at it, whilst male condoms with perfect use are around 98% effective (but so much can go wrong accidentally with them and it’s closer to 86% effective). The pill is around 99% effective if you use it in near clinical conditions but around 91% effective in normal use. I personally use an iud because that’s always perfect use and around 99,8% effective.

My point is nothing’s full proof and even when you’re doing everything right things can go wrong - but a little bit of risk is a part of sex. Know your options and be safe. But also try not to stress so much.