Right on with Hetty and Sandi, Shaw. This is a tropical plant and will die for sure if you leave it outside through a Wisconsin winter. Some Dipladenias (aka Mandevillas) keep a shrubby form, and others, as Sandi said, start sending up twining sprouts and become a vine. You could give it a couple of stakes or a little trellis next summer if you want it to climb, or just prune the long tendrils halfway back to make it branch and stay smaller, like a shrub.
I'd pot it up into a nice new pot with fresh potting soil, and once the nights are consistently below about 50deg. bring it in to the house, and put it near a cool, sunny window. Here in Florida they flourish in full sunlight, so it needs a lot of light indoors. If you keep it rather cool through the winter it will basically go dormant and won't grow huge and take over your house. You'll have it at a nice size next spring once the nights are warm again. It may not bloom through the winter, but will keep its nice shiny dark green foliage for you to enjoy.
Fertilize lightly - half strength - maybe every two weeks and water deeply twice a week. It can take lots of water in summer when the weather's hot, but through cool weather - indoor conditions are "cool" for it unless you keep your house really warm - it won't be growing much so if you start to see the leaves browning on the ends, cut back on the fert and water for a while. Increase fertilizer when you take it outside for the summer and it will bloom wonderfully for you. It's a beauty!
Oh, also it really likes humidity, so if you have a spray bottle of water sitting near it, and give it a spritz whenever you pass by it will thank you. Don't put it near a heat register, for sure! Too dry.
