Period after egg retrieval question

Gi

I had my egg retrieval May 15th. I was expecting AF to arrive on the 29th of May. Instead, it arrived 6 days post retrieval (May 21st). I've read a few posts of others getting AF at a similar timing, so I didn't think anything of it and figured my cycle reset. Today, AF arrived in full force yet again! Has this happened to anyone else? Very confused!