Bertie it could be the Pregnyl, or it could be a faint proper BFP. Especially doing one in the afternoon I'm not surprised it was faint.
I was on Ovidrel shots after transfer, but I still used HPT's several days before my BT - the first was so faint I thought it was just leftover from the Ovidrel too, but they did get a tiny bit stronger using FMU over the next couple of days.
The HPT's never got anywhere close to the strength of the test line, even though my BT came back with a very healthy HCG of 187 12dp3dt.
There is still hope for you yet! I'm wishing the best outcome for you, but even if it turns out to be a BFN this time around, I hope that you can take some comfort in knowing that you have done everything you could to give those embies the best chance possible.