Sometimes a women might not have a period because of stress, weight loss, changing birth control, being on depo. injection, increase in exercise or because of pregnancy. Pregnancy is not that only cause of no-period, so it is smart to talk to your Dr. is you are unsure of what is going on. Plus you could always take a blood test at the Dr. office and see the results, that is the most accurate way of testing for preganancy.
I don't have any answers for you, but I'm in the same boat (almost) I haven't had a regular period for 4 months...no period at all for 3. This past month I had some brown discharge and I took a test...negative. UGH! So frustrating! I don't know what's going on but I have a DR appt on Friday. Hopefully I'll get some answers