I recently got admitted to University of Florida (yay!!) but I got accepted for Summer B 2017 instead of Fall 2017. I submitted the 2017-2018 fafsa assuming i would be accepted for Fall and now I’m stressing out because I need the 2016-2017 fafsa in order to get aid for Summer B and I have NO idea what to do, has this happened to anyone or does anyone know how to help? 
If you need to submit a 2016-2017 FAFSA, just do it. Not a problem. The school will still need the 2017-2018 one to get aid calculations done for THAT academic year as well.
So…just do the 2016-3017 FAFSA…and submit.
It might be a good thing. If you do Summer B (and C?), you can use 2016-17 aid and still have all your 17-18 aid for the next school year. If you attend full time (which may have a different definition in the summer) and are Pell eligible, you might get a good deal of Pell money and yet still get all your Pell for the next year too.
Congrats.