My oldest was 8 months when we got pregnant with our second. I was ok with it, because we wanted them closer together. We weren't trying, but not preventing, either.
When my second was about 11 months old, I was talking to a friend who said you are most fertile right after you quit nursing. Well, my son had just weaned , and I remember having had one "accident" with our birth control. I was nervous, but since I hadn't gotten a period, yet, and it was only one time, I felt ok. Low and behold, the next month, I found out I was pregnant again. I wasn't excited at first-because I wanted more of a buffer between the second and third babies. But,I had three kids in three years- they are all healthy and happy. I enjoy having them so close in age! (Until I have to get them all into college, that is!! EEEK!) LOL!
If you are only using nursing as your birth control, I wouldn't completely trust it. Take a test. Then, test again in a week if it's negative. Just to be sure.
Good luck...or not? :)