You are very lucky to be financially stable and otherwise confident at raising a child at such a young age. Please dont see this as rude, but dont you want to live your lives a bit first. Travel for instance, which is a far more complicated and costly affair when you have kids. You may not think it now but at 19/17 you still are kids. I thought I was mature at 19, but on reflection it is very relative and only when you push on a bit (I am nearly 30) do you realise that you were not as grown up as you thought. I still dont think I am ready for a child of my own, knowing my personality and the things I still want to acheive personally. But I wish you both the best, one benefit to early parenthood is, you may get your independance back when the two of youe are relatively still young as you will have an 18 year old and still be under the age of 40 by a couple of years. However, you should be comfortable financially and know for absolute certain iot is the right thing or you may regret it and resent each other, with the next 18 years being ver difficult. Who are we to judge.
To answer your question - what is happening to your body is quite normal when you first come of contreception, a false or mimic period can occur out of cycle. I has happened to my partner when she came off the pill mid cycle and lasted 3 days, fairly heavy.