AF after mc?

Megan • Happily married. One 2015 angel baby, little girl born Sept 2016, & one due June 2019.
It's been exactly 4 weeks since I started bleeding, which lasted about a week and a half. Yesterday I had felt like my period was coming on but figured it was a cruel joke because my hcg was still at 60 last Thursday and the doc said it might be another week until it's back down and then four weeks (at least) to get AF. I've been having brown discharge off and on the last week. 
Anyway, woke up this morning and I think AF is here. Cramping and light pink. Has this happened to anyone else this quickly...28 days on the nose? I figured I'd have a lot longer to wait. So confused.