If her period's not due for two weeks it's quite possible she's ovulating about now, so you really need to be using protection every time if you're trying to avoid pregnancy and STIs.
Also, you should know that when a woman has sex with a new partner, it can affect her cycle for the first month or so. This is especially true when she hasn't had a lot of sexual experience and is quite young. Don't be surprised if her period comes a few days late, especially if she's been stressed out.
It sounds like you were just a little too rough this time out and things just got a little irritated down there. It takes a young woman's body time to adjust to having sex when she starts out, and time to get used to a new partner. I agree with Clysta, pay attention and try being a little more gentle till you get used to each other.
Implantation bleeding doesn't last for 6 days. IF a female gets it, it is extremely light and usually does not require use of feminine products. It sounds like irritation - brown discharge is old blood that the body is expelling. So it could be leftover from her period, and sex just made it dislodge. When you have sex, make sure to take it slow and build up. Listen to her and if she's in pain stop or be more gentle.
Help please I'm so stressed