My last period ended on September 5th, and I'm not due for my next period till the 1st of October, on the 7th I found out I had a UTI which I get quite frequently. I took my medication and it went away. I had protected sex on the 10th and 17th with my boyfriend and after having sex on the 17th, the 18th I started experiencing brownish discharge that hasn't stopped and has began to turn into a light pink like there is blood in my discharge. I'm not on birth control and the past couple of days I've had minor cramps and it's itched a little down there. So being my worried self, I took a pregnancy test which expectedly came out negative. Please help me to figure out what this may be. I'm 17 and I am NOT ready to be pregnant.

I say this with all respect, if you are not ready to be pregnant you should not be having sex. Since sex equals babies.

And if you are having sex why are you not on birth control and using condoms.

Condoms are far from 100 percent effective, One expert on here rates the failure rate as high as 13 percent or so, My research shows the failure about 5 to 8 percent. But that is still good odds of sperm being present, so that means it is easy for you to be pregnant.

Next yes it is way to early and what you are saying could well just be an infection,

You are on the waiting game now. Pleaes ask any follow up questions by answering your own question, and/or come back and let us know how it turns out

I wouldn't say that's it's cramps that I'm experiencing. More of feeling gassy. And I don't feel the need to pee all the time, nor do I feel sick at all. I've lost 5 pounds in the past week most likely because I haven't been eating much because I've been stressed. I talked to a friend and she said she had the same thing, it being an infection. So I'm not quite sure.

Infections, stress, air quality, wheat, laundry soap, you name it, it has the potential to affect your cycle. You stressing out right now could be affecting it. Calm down, breathe, and let some blood back up to your head.
I really didn't want to be Captain Obvious, but I couldn't help but notice that you said you get UTI's frequently. And it was after you had sex that you've noticed these problems. Could it be possible your boyfriend is giving you the UTI's, which may not of caused a lot of trouble for him, so he never got treated?
You also mentioned itching, discharge, I'm guessing a different odor as well?
If you took antibiotics to treat the UTI it's a very likely chance that you now have a yeast infection. The body has natural antibodies and natural yeasts (picture a scale w/ A's and Y's on either side). With an influx of antibiotics, that's weighing down the A's side, the Y side is up in the air going oh no, we need to be balanced again, and hops into overdrive. So even after you've finished the medication, the A side is somewhat normal again, the Y side went into hyperspeed and has pretty much taken over. Y side in charge you are more likely to experience itchy, redness, swelling, smells, discharges, etc... which can get pretty icky if left untreated, not to mention it is highly transmittable to a sexual partner (OUCH I might say) if partner doesn't already have itchy drawers.
I would worry about a possible yeast infection or a possible re-infected from boyfriend before worrying about possible booties knitting in your future hon. Make sure he get's checked out for either infection, and go back and see your Dr so they can get you on the right meds to get you back healthy again.