Could I Be Pregnant?

(I apologize for how long this is, I put quite a lot of details in this because I'm really worried and thought with the more details I put it'd be easier to recognize if I am or not pregnant. I really would appreciate your answers as soon as possible, you have no idea how much this means to me! So if you could spare ten minutes of your time and read this over, I'm scared and I know I'm not ready to have a child so I want to know in advance if there's anything if I should be doing, or if I have an STD.... This is so scary, please help me!)

I'm seventeen years old, and my last period started on May 20th, a month and two days ago. My period's are usually on time, and are three weeks apart.

I had unprotected sex on June 5th, and my period was supposed to start on the 15th of June, but never did.

It's now the 22nd of June, and I have gotten a brown discharge and more string-like white discharge which I've never gotten before (I've only gotten white thick discharge but never string-like).

I have done some research on my own, and I've found two things that have concerned me: One is that I could be pregnant, and that brown discharge is an early sympton of pregnancy. I've also gotten headaches and pains in my abdonimals, like when I have my regular period.

The second thing I've found was that I could have an STD, which scares me even more than being pregnant.

I really need reassurance, I'm also a depressee, and such. Being pregnant could also cause complications for me because I'm young, and my family won't approve.

If I am pregnant, I'll most likely be caring for the child by myself or have help from the father's friend, because the father of the baby (if I'm even pregnant) is moving away for school.

Do you think I could be pregnant?

My mom has my health card, and I don't have any money at the moment. I don't want to openly tell her that I could be pregnant because she'll have a hard time accepting it, and she doesn't know for sure that I lost my virginity - have family issues, not much communication or support.

I'm also not on any form of birth control - I've been meaning to get the shot, but I guess I couldn't wait (stupid idea, I know).

I regret some of the decisions I've made, but I absoloutely don't want to have an STD (we've used condoms a few times, but the second last time we had sex he didn't have a condom on for a good while at first).

The guy I had sex with claimes he has been "checked", should I ask him about it more thoroughly just to be sure? Or have him get checked again? I don't know a lot about STD's and what you do if you have one, I'm really scared about this whole thing, and I really would like someone's advice.

I was planning to go get a pregnancy test with the guy on Monday, but he has to work, and I don't have any money.. I have no idea what to do. Please help me.