D.P.
1. I would wait a few days or a week, and if you still haven't gotten your period, do another home test, or call your ob.
2. You can become pregnant while nursing so you should be using some form of birth control, unless you're wanting to become pregnant.
3. It's possible your hormones are just out of whack, so don't panic. Just give it some time, but if you're stressing about it, call your doctor, as you don't need the extra stress that also causes your cycle to be affected.