Hi - I do know it's possible to get pregnant while breastfeeding a 3 mth old, as several people I know have done so. I also know you can spot while pregnant as I did with both my pregnancies (my son and my m/c). Generally spotting is old blood, and not fresh red.
However, it's possible you aren't ovulating yet, and just getting random hormonal spotting too.It varies so much from person to person! With my son I bled for 5 weeks post-birth, had a week off, then had a period. After that I didn't have anything until he was 7 mths old.
I guess a pregnancy test is the only way to know!