A similar thing happened to me was on 50 and a 25ug patch and switched to a full 75 patch and it was as if my E levels had dropped. I rode it out and it settled. There is no way that it could be a bad batch as they are rigorously tested and have to comply with a strict specification and each batch is assessed and passed by a Pharma QP. It also could be that the manual cutting could have made a slight difference, as to cut exactly into half may be v slightly different each time. Or, it could be as Wombat states your natural cycle kicking in.
Hope this helps a little,
Rhiner