Hello. I agree that you should take a pregnancy test but it won't be positive until 2.5 weeks after you had sex. Was this your first time sexual experience or had you been having sex before that. I will throw in now that you need to be using a condom. Could you be getting your period? When was your last period by the way? Brown discharge is old blood. Spotting is pretty common. To me, you sound like you really need to go see your doctor. Is that possible? Does your mom let you see the doctor without her present?
There are a lot of things this could be. If you had been having sex already (not just three days ago but before), you could be pregnant. Have you taken a home pregnancy test? They're really reasonably priced and quite sensitive at the Dollar Store. Or, you could have a virus. Or, you could have anemia (low iron). Or, your period could be coming. Write back if you have ruled out all these things.
If you are thinking that sex three days ago has resulted in a miscarriage, no it didn't. When was your last period? And was the sex three days ago the first sex you have ever had, or did you have sex before in this cycle?
oh and yesterday, I was laying on my bed and out of no where I felt this burning sensation? on my chest and throat. I'm not sure if I should describe it as 'burning sensation' but my chest felt like it was tightening. As well as my throat? Not sure how is that possible or why it even happened. It's happened times before out of no where but I haven't felt that pain is so long. Why now?