bfn but still no period

lindsey • Currently ttc my secound child. 25 years old from england with my partner for 12 years and have a 4year old daughter x
Just wanting some advice af is one week late cd37 today but had 2x BFN both a week apart.Booked in for blood test on 19th july if Af hasnt shown yet. Has anybody ever gotten BFNs on hpt tests and then a posotive on blood test?