Hi hayldrew,
sorry to hear this is so stressful for you (congrats on 11 frozen embies though).

I've had a slightly similar experience- my first FET was a natural cycle ( ie no meds just using opks to track O), I had a transfer then that night i noticed a small amount of brown spotting. I got this same spotting everyday throughout my tww, some days more, some less. I rang my clinic and got a progesterone level done which was low-ish, the nurse thought that might have been the problem, but later my FS ( who I have now left) said everything was fine. Anyway, I eventually got AF and a BFN, which I was expecting of course. I was REALLY upset as I knew there was no hope for the cycle and i had basically 'wasted' the embie, as there was no hope of it implanting if my lining was already shedding.

I know this situation isn't entirely the same as yours, but I would be hesitant to go ahead with a transfer if the bleeding continues, I doubt they would recommend it anyway? Its VERY frustrating to have a cancelled cycle ( I've had 2) but if the signs are there that it isn't going to work I would prefer to have a go when there was a better chance of it working. ( after my last cancelled FET cycle I got a BFP the next month, in retrospect I feel that the first month it wouldn't have worked cos my lining wasn't up to scratch)

I've never had spotting before ovulation or while on puregon so can't comment about whats causing it, but I hope it either stops, or they work out what the problem is! So sorry you are going through this, it is really one of the hardest parts of ivf, the constant frustration of things not going right.

Big hugs to you, I hope you get your bfp really soon!