You can breastfeed straight through a pregnancy with no harm to your nursing baby, and if you want you can continue to nurse both children as long as you are first meeting the second baby's needs.
You might not have regular periods yet, so if your tests are negative for now, continue to eat healthfully and "act" pregnant just to be safe (keep taking prenatal vitamins, which you are probably on for nursing anyway), avoid alcohol, and use a menstruation chart. I took a pregnancy test every month after each of my children because I did not have periods for almost/over a year, was on the mini-pill, and did not want to be surprised. If you still aren't regular and have any other signs of pregnancy, test again or see your doctor.