Question (and a little bit of story time)

My friend who is currently 19, has a 3 year old son who is absolutely adorable. Well, the thing is, back when she was 16, she was throwing up all the time and just felt really awful so she was getting tests done for cancer and such and they didn't test her for pregnancy because she was having her regular periods. Turns out she was 6 months pregnant and had her son 2 months later. So my question is, can you have regular periods that follow your normal cycle when pregnant? I know if you look it up they say its not really a period but I was just wondering if it has happened to any of you.