Is it common for women who haven’t had a regular period postpartum to bleed after sex?

I have yet to have a regular period due to breastfeeding my 16 month old, I will randomly have one and then nothing for months. But Sometimes after sex I’ll start bleeding, and it can last for days. Is sex triggering a period? I went to the obgyn about it and she just assumed my period started, but nothing about my body is normal right now so I just want to know if this is common.