Found something strange in my underwear? 😳

My period has always been so irregular but as years gone by it has been SUPER irregular. Like once a year AF decides to show up. This has caused infertility issues so with our last child we took medicine to conceive. Anyway, supposedly you don't get periods while exclusively breastfeeding or whatever. But I just had my period literally 2 weeks ago In like over a year. (I was pregnant) But I went to the bathroom and I seen this on my boxer shorts wtf?! What is this ladies? Sorry I'm embarrassed 😩