Used to get A's, now C's, what's wrong with me?

I see what your problem is, the same thing is happening to me...So, first try to see if there is any added stress in your life, if there is, don't think about it as much. Second, practice more with your subjects. It doesn't always matter if the study habits you had before worked, with the harder material you may need to chang them. Try a different type of studying, eg. flash cards. Also, for example in math, you can check to see if you really have the skill down, you do this by finding an online calculator service and you give yourself a problem. If when you put the problem into the calculator your answer is the same over and over and over and over and over again(100% of the time) you have the skill down pat. Hope this helps!!!