Light period 5 days after sexual activity - Is this implantation bleeding?

my boyfriend and i might have engaged in some risky behavior on 9/4. this was actually the first time ever...we have never actually had intercourse, but when my boyfriend ejaculated, i am worried that some semen may have touched the outside of my vagina. on friday, 9/8, i experienced some cramping, similar to menstrual cramping, but no bleeding. i thought it was just an onset of my period...

however, on 9/9 and 9/10 (today) i have experienced some light bleeding...much lighter than a normal period. could this be implantation bleeding?

also, the last period i can recall was at the end of july, and i am fairly regular.

could i be pregnant? can there be any other causes of light/spotty periods? coudl implantation bleeding occur as soon as 4-5 days after conception?

thanks so much in advance