I have used Fortel Extra a week after I got my first +ve. It was strong and +ve.
I always had problems with Fortel Ultra though. I was 5 days late once and I got a +ve on their brand (I always used FMU and a sensitive test so I would be squinting to see the line)
Anyways, a second line came up straight away. I went to the DR's told him, he did BT came back -ve. Did another test (more expensive one) and it also came back -ve. Period came three days later.