My gf has irregular periods and she does not have a period at all unless she is on birth control is that bad?

Good question: Thanks for being concerned about your girlfriend. Her irregular periods off of birth control likely means she is not ovulating for some reason. There are lots of reasons for anovulation or irregular ovulation in the late teens, and she hopefully has discussed this with her ob/gyn. Using birth control is one way to try and get hormones/ovulation back on track. It's not bad that she's not ovulating.