Failure rate of combined birth control pill is less than 0.5% on regular use. You need to check your pregnancy
status immediately after missed period through home pregnancy test or blood for beta-hCG test.
Other reasons are side effect of pill itself, indigestion
imbalance, mental stress, use of certain medication etc.
Take healthy diet with iron supplement, use some anti-emetic drug, drink more water, eat green vegetables & fresh fruits and take sound sleep.