Have you had a blood test done? (i suffer from low blood pressure also)
sometimes pregnancy tests don't read that you are pregnant until later on.
If you haven't had any bloodwork done to see if you're pregnant than I suggest you ask for one. Either way it would be helpful to find out.
If you are pregnant maybe it's not the best time to be in a weight loss program (especially if it's one that deprives you of some things that you might need during your pregnancy) and depending on your diet it might throw your body off a little and make you feel a little dizzy more often.
I really suggest to have bloodtests done and ask your doctor for an ultrasound. If you are pregnant you need to know soon, and if the bleeding is being caused by something else than you also need to find out soon.
I really hope you get answers (i know what it's like not to get answers from the docs)