Don't listen to the people who say they don't get a positive for 3 or 4 months. Read the directions for the test. They will tell you how soon after your missed period you can take the test for the most accurate result. I would guess that the women who didn't know for 3 or 4 months either didn't test correctly (didn't read the directions and follow them) or just didn't test at all.
I think you just have really irregular periods. That's not all that uncommon. Stay on the provera to regulate them and you should have fewer issues.
I think it's great that you want to wait to have sex until you're ready. When you have it is totally your choice. If you want to wait for marriage, that's great. If you decide sooner because you really love the guy, that's fine, too.
In the mean time, good luck with school. I really don't think you have anything to worry about pregnancy-wise.