Exclusive breastfeeding is the oldest form of child spacing, so it is not unusual for you to not have had a period yet. However, a pump does not act the same as a baby, and most mothers who rely on pumping find that their menses returns sometime before their baby is 6 months old. However, because your body is still providing all of the milk for your daughter, your body's return to full fertility is likely to happen gradually over time. There may be times that your body tries to ovulate, and you experience EWCM, and then it goes away without you actually ovulating. This is caused by a surge of estrogen that is not enough to get over the threshhold to actually ovulate. You may experience spotting as your hormones fluctuate. You may even experience a heavy bleed that looks like a period that is caused by a sudden decrease in certain hormones before you actually ovulate. It is common for your body to try to ovulate several times before actually ovulating, but you have to assume that you could ovulate any time you have any kind of CM, not just EWCM.
When you do start ovulating, it is common to have one or more cycles with a shorter than usual luteal phase. Many women experience quite short luteal phases that could not possibly sustain a pregnancy. Others are able to get pregnant at their first postpartum ovulation before their first PPAF.
As for what you experienced, I can't tell you for sure what happened. It is likely that your body was just trying to ovulate since this is the first time you experienced EWCM postpartum. It would be unusual for your body to have successfully ovulated the first time it tried, but it happens. Just ask a mom with kids 10 months apart. The spotting you describe is consistent with what could be observed during regular hormone fluctuations postpartum or implantation bleeding.
I would consider taking a pregnancy test in a few days to a week (though I doubt that you are pregnant), and I would start charting immediately. These are obvious signs that your fertility is returning, and if you do not wish to get pregnant at this time, you need to be cautious.