Aside from pregnancy, why would you have menstration 11 weeks post partum, and not again (now 20 weeks post partum)? not breastfeeding.

Can be normal: There can be variations in a woman's menstrual cycle that are considered normal and not to alarm about. If these changes become consistent and constant, then evaluation with a provider can shed some light on a problem.

Irregular periods: From your question, you've basically had 2 1/2 months without a period. This can be common, but at this point likely hormonal and not due to prolactin (hormone of breastfeeding). Of course, you may be pregnant again, so check a test if that's a possibility. Second, your estrogen/progesterone balance may be off and that's why you haven't cycled. See a doctor if you're worried.

Time for evaluation: There are many potential reasons for amenorrhea (lack of menstrual periods) and there is a fairly standard evaluation process. If you are five months after delivery on no hormones and not breastfeeding, it is time to see your gyn to start the evaluation.