Why would I be getting cramps?

Kat

I had a five day embryo transfer of two early blastocysts last Tuesday 22nd August and yesterday had some light brown spotting on and off throughout the day and then a bit this morning. I also this morning for half an hour or so had some cramping down below almost like AF cramps? I'm wondering if this could be AF or could it be implantation this late? Due to take pregnancy test this Saturday. sorry for pics but this is the spotting I received x