Don't panic-DO keep testing, but don't panic. With my first child, I didn't get a period until a month or two after I stopped nursing. With the second one, it started four weeks after delivery, was regular for a few months, and then went crazy for months--it was far apart for a while (once I went more than 80 days and felt like I was going to explode from the fluid retention). I started buying pregnancy tests in multi-packs. And then it came every 15 days for a while, which was a huge pain. Check in with your OB-I remember mine ended up having me get a vaginal ultrasound to be sure there wasn't anything left in my uterus from the delivery that was messing with my hormones. There wasn't, and I went back to normal soon after that. Good luck!