I have a 3 month old baby. I got my period back when he was 2 months old (even though I'm exclusively nursing). That first period lasted 14 days then stopped for 4-5, then I had more bleeding for another 2 days before it stopped. Then on day 28 of my cycle I had some spotting. Thought my period was about to begin, but nothing happened again until day 35. On days 35-39, I had a lot of stretchy mucous with occasional light pink spotting or a couple times, some red streaking in it. It was never regular bleeding, though. The spotting and streaking is seen on the toilet paper only--I wouldn't even need a pantiliner if it weren't for the mucous. My husband and I had unprotected sex on day 24. I took a pregnancy test on day 35 and it was negative. My normal cycles are 26-31 days long, but this is the first one since the birth of my child, so I don't know what's going on. I've had two other children and my periods resumed normally after they were born. It's now day 42 and still no period. How could my period resume and then disappear like this? Any thoughts?

Pregnancy will be a possibility, but since you are nursing exclusively there is a good probability that it is simply your body adjusting to the hormone changes with birth and breastfeeding. It may not have even been a period that you experienced, given the timing and duration, but still part of your postpartum recovery.

If you are not hoping to be pregnant just yet, while exclusive breastfeeding can be fairly good at preventing pregnancy, it is not 100%, so you may want to consider using some sort of birth control.

Is there any danger to waiting this out? Or might there be some cause for concern that I should notify my OB about this?

There is no danger in waiting this out, it's quite common actually, particularly while breastfeeding.

Now, just a warning, I wouldn't even say breastfeeding is 50% effective in preventing pregnancy. I know of several people who got pregnant during the first months after birth and while breastfeeding. I actually see it all the time in the hospital I work at.