What Causes Delay In Periods Despite Having Negative UPT?

I am a 35 year old and taking birth control pills. This last pack I missed a few pills and am now about 5 days late on starting my period. I have taken two over the counter pregnancy tests, both negative. I am supposed to start my new pack of pills today, but don't know whether to go ahead and start them or wait to see if I have an actual period.