Sooo I feel I have to ask this because I am wondering if this is weight related. For the past 2 months I have been having menstruation or btb every 2 weeks. I am on my 3rd cycle in less than 2 months and it is getting a little old.
Full disclosure I changed birth control pills 3 months ago from a generic to the name brand. Same core ingredients supposedly.
I also have lost 18-19 pounds since July.
My obgyn thinks it is just the pills, my fiancé thinks it's the weight loss. I don't know what to think but it is aggravating.
Any thoughts on this?

I've only heard of weight loss and the stress it can put on your body as being a cause of later periods or even can cause you not to have one. Your obgyn probably knows what their talking about since they have experience in this field and I'm sure others have probably had the same issue as you.