positive test after stopping birth control

Asking for my sister. She stopped her birth control on June 20th. She had some spotting on the 21st and 22nd. She never got her period though. She took a test 2 weeks ago and was negative. She took a digital today and it was positive! So my question is when did she ovulate if she never had a period? I'm 5 weeks 4 days pregnant and we're trying to find out how close we are but no dates to go off of. Thanks!