Taking pregnancy test on birth control

Is there a certain day or time that I should test, when on birth control? I'm on the pill but I'm concerned about pregnancy because I threw up and I've been having sore nipples. Nausea isn't uncommon for me, but my nipples are hardly ever sore and I usually throw up when pregnant or drunk. I'm on CD 21 but my withdrawal bleed lasted somewhere around 9-12 days and it's been like that for the last 3 months. Thank you for taking the time to read and reply, I really appreciate it :)