Is she eating more solid food in the last two months? If so I would think that's what' s causing it. Make sure she gets plenty of water every day, the heat will dehydrate her and make her poop harder. Also, fruits and veggies are important, along with no processed foods. And let her get plenty of exercise, walking, crawling, climbing, etc., to keep things moving through her system.
The straining is more than likely due to solid poops that she may not be used to, nugget size wouldn't cause a need to do that, unless she's already constipated and it's just coming out in small pieces. Does her abdomen feel rigid, as if her colon is full? If so make sure you give her pear or prune juice every day to get things going, and continue her on it as well..