My fertile days are from 1/8-1/13 my ovulation date is 1/13. My last period was 12/31-1/5. Me and my partner had intercourse on 1/8 today is the 13th I've been having period like cramps in my abdomen today went to bathroom wiped it was light pink. Could I be pregnant or is this implantation bleeding? I am confused about what is going on. I took my BBT around 7:30pm and it read 98.56F I've also noticed last week waking up a couple of days with low back pain...

No real signs for pregnancy 5 days after sex, spotting perhaps, but that is also from 100's of things.

Really no way to tell until you either have or miss your next period.
You have to wait about 10 days to 14 days after your missed period to do a pregnancy test