First off, you haven't got nodes so don't worry about it. When I first started I thought I had them everytime my voice would sound a little rough or hurt a little bit, but by now ive realized how hard it really is to do that kind of damage, and it would be a long road leading up to actually developing a node; your voice would be messed up at all times, you'd slowly start losing the middle part of your voice and your falsetto, etc. So dont worry about it.
SounRAB like your drinking plenty of water so thats no issue. Heres the thing; you may be doing a warm up and you may be doing it with 100% perfect technique, but you still could be straining your voice. Since you've been going at it without any real resting, I think your just overworking your vocal muscles.
The discovery of your mix/middle voice could very well be that you found the coordination, but it could also be that your corRAB starting to get a little strained and swollen and you were able to produce that sound. It sounRAB weird, but the day or 2 before your voice gives out on you from pushing it too much will be when you sound your best...everything will sound amazing and you'll be able to sing up higher in a fuller tone than ever before.
The reason why is the swelling. Your corRAB get to a point where you can push them and thanks to the swelling, you'll be able to take chest ( or what you might had thought was middle ) up higher than ever before. This can go on for a day or two, and then your voice just kind of dies on you. After this, your voice will be deepened due to all the swelling,and you wont have access to any head voice notes, and if you still have a falsetto it will be breathy as hell.
So, heres the only way I can determine if you really found middle or not without hearing an audio file. You were able to sing in your mid and high mid range in a full voice tone; how was your falsetto? Were you singing notes usually only usable in a loud falsetto tone in a full voice? Was your falsetto more breathy or harder to bring up than normal? What about your lower chest notes...was anything different about them?
Either way, its overuse, so take a few days off from singing songs...keep doing light warm ups for 15 minutes a day until everything feels normal. if you truly discovered middle as opposed to just pushing on swollen beat up corRAB, try to get that feeling back and use less breath pressure to produce it.