Hi there! So, I also have PCOS - was diagnosed at the age of 14, but also recently had that diagnosis confirmed (as I wasn't sure that the parameters my endo from so many years ago were legit. Guess they were!).
Liana has some REALLY great info in the post above on the scientific side (Thank you!), so I won't go into that, but I will go into what I personally have seen over just the last three weeks that I've been on IP (which have been a bit of a roller coaster).
I had PHENOMENAL losses the first week- nearly 10 lbs. Then had less than a single pound loss the second week. The third week, back up to ~6 lbs down. Depending on when you started the BC, that could be causing part of it. I attribute the lack of loss over that second week to a few things:
1. My endo put me on Metformin. My blood sugar is fine, but she wanted to regulate my cycle, and I don't tolerate BC at all. My body didn't know what to do with the hormones, etc. that were new to my system. I was a hot mess that week - nauseous and not hungry at all most of the day and then I'd be STARVING and carving all the carbs in the world (which was weird b/c I had sweet/sugar cravings before IP, not potato cravings...). Once my body went "ok, I think I know what's going on here," I evened out.
2. I wasn't eating enough, so my body went into starvation mode. Now - a couple of things to point out here. I have A LOT to lose, so the three packets a day + protein + veggies was probably not quite enough for me anyway. This is not the case for everyone. But add in the fact that I have an 8 month old lab puppy that has to be walked a few times a day, and I really wasn't getting enough in. I wasn't feeling weak or all that tired, but I do think my body freaked b/c it wasn't getting enough calories in. I have started adding in a third packet every day on days that I walk for more than 20/30 minutes. Or - and you'll be happy to hear this - on days that I weight train. Agree with Liana - you probably won't be able to go as hard as you used to, especially in the beginning. But give yourself time, and slowly work it in, and you'll definitely be able to get back to doing those kinds of exercises.
I guess my point to the above is - don't necessarily equate the lack of progress at the last WI to the diagnosis or the BC. As for the diagnosis, you've likely had PCOS for awhile, so the 3-4 lbs were happening while you had it. As for the BC, you likely need to just give your body time to adjust and find its new normal. You may not be back to 3-4 lbs, but 2-3 lbs is still GREAT progress. I've NEVER had a diet before that helped me lose the kind of weight I'm losing (even in just three weeks!) with IP. I really believe that once everything normalizes, you'll start to see results again
Hope this helps!
Leslie