You were most likely pregnant before.
If you ovulated 5th, fertilised 6th, implantation is not likely until 11th at the earliest and test not likely to show positive until 19th. So unlikely!
However you don't give any details of your cycle.
- Last Menstrual Period start?
- Cycle length?
- Dates of sex with bf since LMP?
How are you going to tell your bf you cheated on him? He will probably find out anyway, even if you don't!