SPOTTING OR ??????

Lyzette • | Momma of 2 girlies <3 | 20 | Wifey |
I had unprotected sex , Five times my SO came in me .
First day we made love was Sept 14 , Sept 17 ( Day before ovulation ) , Sept 18 ( 1st Day Of Ovulation ) And Sept 21 ( 4th day of ovulation) And finally Sept 22 . My last period was Sept 2-8 
Today around 2pm I felt wetness in my underwear so I went the restroom and saw blood ? No cramps at all but little back pain hours earlier. It wasn't bright red it was brown when I wipped , was very little like a quarter size . Never never ever ever had this happend to me . I'm suppose to get my next period October 7 
Could I possibly be pregnant ? 
When is the best day to test ? 
Should I buy a cheapie now and test ?