Are you feeling better yet tonight? If it's not getting better, see a doctor tomorrow (sorry, I'm teaching med students this semester, so am prone to thinking of things like referred pain from more serious problems, which it probably isn't, but I can't help from coming to mind). I did wake up with some horrid crick in my neck once like that...I was a teen at the time, and just moving my head the slightest sent shooting pains through my neck, and raising my arm to brush my hair nearly brought me to tears. I don't know what I did, but it was excruciating (never before or since then have I experience something so painful), but then it was just gone again when I woke up the next day. Hopefully, whatever you've done to yourself goes away as quickly too.
Low potassium levels can cause muscle cramping, but usually you'll first experience that in the legs. Nothing wrong with eating a banana anyway, but if your potassium levels were so low as to be causing muscle cramping, a banana isn't going to be enough to correct it...and it would be good reason to see a doctor to check that out. Unless you are on some crazy diet that might be depriving you of nutrients, you'd want to know why your potassium was too low (it is a side effect of some high blood pressure medications, if you're on any...if so, it would be wise to get that checked; the doctor can give you potassium supplements or possibly change your prescription if that's the problem).
Never mind.