Hey Frankie,
I had my HSG done several months before the first IUI and I didnt thing it had any bearing on the BFP at all. I also had a polyp removed beforehand too.
What I thought may have been different was after the FSP treatment was that I had (TMI) an awful period, passing huge clots and very heavy (had to change tampon & pad every hour) - very unusal for me.
We then took a break from Oct to Feb and I think probably cause I was more relaxed and knew what to expect (and I had happy drugs) that made it all the more comfortable.
I'm not saying that having the HSG might be the factor here for you - I just dont think it was for me.
Good luck