Implantation bleeding?

[Sarah24601]

Hi,My girlfriend and I are on our first round of IVF. I'm 6 days post transfer. Earlier this week (2 days post transfer), I had awful lower back pain. It was there all day then by the next day it was gone. Had a few stomach cramps but none since 4 days post transfer. However, yesterday (5 days post transfer), I noticed a dark brown discharge. It's been 24 hours and while has become a slightly lighter brown, it is still brown and is slowing down. It's a real rollercoaster as we're just unsure if it's going to be my period, a result of the pessaries or implantation bleeding. Has anyone got any thoughts on this please? TIA x