Can my sister still be pregnant?

My older sister has recently confided in me that she might have to make a doctor's appointment because her always on time never late and always regular period cycle hasn't came and it's been three weeks. She hasn't had since April and believes that she did have her period in May but she isn't too sure because according to her calender she forgot to mark it down for that month. She can only rememeber that her last period was really painful where she spent most of that week and a half in bed and she bled a whole lot more than usual but thought that every girl must have atleast one really weird period in their life so she blew it off as unimportant. I don't think she's pregnant, but would it be possible? We researched it and found something about girls bleeding and then finding out they were preggers and had lost a twin. I'm scared for her but I'm sure we r only reading too much into this. Any thoughts on what else can be going on?