How many days late can your period be before you start to worry? I'm always every 22 days, but now it's day 27.

Up to a week. Periods can vary somewhat and sometimes be later due to illness, travel, weight changes, a lot of exercise, etc. If you have had unprotected sex during your most fertile period (from about day 5 to day 11 of your last cycle), then you should start checking your pregnancy test starting on day 29, particularly if you have nausea, nipple tenderness, frequent urination, or fatigue.
2–3 days. If you have regular periods, a positive pregnancy test could be possible 2–3 days after a missed menses. Other common causes are delayed ovulation.