Thank you for writing.
If your test is negative, it's not likely you're pregnant but I suggest a blood test at a clinic. Breastfeeding a three year old who has other food for sustenance means you're not "full-time" nursing like you would a baby so you wouldn't be as protected from pregnancy as a newer mother.
If your blood test is negative, other reasons for being late are lifestyle changes, stress, medications or a change in body chemistry. If this irregularity continues, further testing can let you know if you have a hormonal imbalance.