Period feeling even though pregnant

I've heard people who have been pregnant say that they still felt like they were going to start their period before they found out they were pregnant. Why is this?? What causes you to still have that period feel? (Bloating, pressure in abdomen, moist feeling in vagina, just feeling like your period is gonna start any moment?) I never get period cramps though. And how do you tell the difference in period coming or pregnant? Can someone answer this?