Is my SO pregnant?

My SO and I slept together 8 days before her expected period. She took a test the first day of her late period. Negative. She took another test the day after. Negative. Two days later (the 4th day of her late period), she took another test. Still negative. We aren’t planning on having a child atm but we’re wondering if she could be pregnant. What the likely hood of her being pregnant 4 days in to her late period and 3 negative tests? When should we test again taking into account it’s been 12 days since we’ve had sex and we’re now 4 days into her late period.