I had sex, then I got my period on time, but it was only for 3 days. & then a couple days later I had what looking like to be spotting it was a brownish red color. I'm having pains on the left side of my stomach & headaches all the time. I'm always having the feeling I'm about to throw up, but I won't let myself. I'm cramping really bad too. Could I be pregnant?

You can always be pregnant if you have sex.
You can have a STD.
You can be sick from something else.
So... only a doctor or nurse can determine this in person.