I've been a vegan for almost two years now, and I did lose about 5 pounds when I first started. I'm leaner than I used to be, but definitely I did not lose too much weight. I'm pretty small naturally, so other people I have heard can lose up to 10.
I'm not sure about the bowel movements exactly, but I haven't had a problem with it so maybe it does help with more veggies.
As for periods, I don't get mine at all. Though this is due to starting the pill a year ago and not getting them while on it, so my doctor has told me to skip the placebos and takes the packs straight through. I've been vegan the whole time I've been on bc and my period has not come back, even trying the placebos once in a while to see how my body reacts.
My reccomendation is to get a vegan/vegetarian multivitamin to make sure you're getting all the nutrients you need, but of course they'll be from all plant based foods. I use
Rainbow Light brand. Mine are called the Just Once (the ones with iron, though they also make them without) multivitamin. On the back it says veganguard at the bottom, which means it's totally vegan.
I would try the 30 day veg pledge, I did two years ago and that's what got me so interested in vegetarianism and then a month later veganism. I'm not trying to preach, so I hope I don't come off that way, I just think it's a great thing you wanna try out being a vegetarian. If it's not for you, you'll realize it. If it is, even better. I did have some cravings at first, but I was doing it for myself only and not to prove anything to other people, and I stuck with it. This isn't the case with everyone, but believe me 30 days goes by fast.
If you'd like suggestions for easy recipes, or store bought meals/snacks that are vegan or vegetarian, just shoot me a PM and I'd be happy to help.
Sorry I couldn't answer all your medical questions, try doing a search on theppk.com which is a forum site like this. I believe you have to be a member to search, but you'll find great answers there.