3 Weeks Late

I'm 3 weeks late today and I've taken upwards of 10 home pregnancy tests. All of them say negative. I started spotting about 4 days ago. It's only when I wipe after going to the bathroom and it's a dark red/brown color. Not enough to warrant a panty liner and not the usual red color. I've never been more than 2-3 days late before and I've been feeling lightheaded recently. I went to get a blood test today, but no one has called me with the results yet. I'm just wondering if anyone has had this happen? I stopped taking my birth control in August and I've been taking prenatal vitamins for a month or so. Maybe my body is just now reacting to being off birth control? Anyone have any similar situations?