Pregnancy test when taking the pill?

I may have made a mistake in the first week of taking the pill and had sex during those days. Exactly 3 weeks after intercourse my withdrawal bleed came as usual (the same strength as always). Does a period always come straight away if you are pregnant or is it possible in my case that it doesn't come after 3 weeks and only comes in the following month? I want to do a pregnancy test but I'm at a loss because it's always said that you should test after you miss your period, which isn't the case for me. That's why I'm worried that the test won't work after 4.5 weeks because I haven't missed a withdrawal bleed. I've heard that a lot of women have had their periods despite being pregnant and I'm really panicking right now. 4.5 weeks have passed since I had sex and I don't know if I can test with certainty.

Can anyone help me?
LG