according to your description the possibilities are,
1.normally exclusive breast feeding will lead to amenorrhea
, which is natural contraceptive
2.you didn't mention how many cycles you got after delivery. Sometimes first few cycles will be irregular after delivery, that may be the reason for your delayed periods.
3. there is possibility of pregnancy
also as you are having amenorrhea after already established periods.
Now you take urine pregnancy test depending on the result you can proceed after consulting your gynecologist.