@Health&Music my first impulse would be to ask "why, oh, why did you do that?" but at 23 we've all done questionable stuff so there. I am probably going to give you a little more information than you expected but let's start with the obvious: If you do something different to what you are used to doing and that causes something new to be experienced it is most likely the cause.
Bodies are made to be stable. Homeostasis (i.e. the ability of the body to maintain a stable internal environment) is how we survive. In order to do that in an ever changing external and internal environment they have in place a massive number of complex processes that affect cells and tissues and, by association, nerves. Everything we do that pushes us is a stressor. This includes fasting, exercise and any repetitive activity like, in your case, playing the guitar. Stressors create adaptations in tissue and those adaptations happen through inflammation which is the intermediate stage between tissue (muscles, tendons or nerves) that in untrained or unused to the demands made on it and tissue that is trained. That intermediate stage however presents a conundrum because inflammation further detrains tissue so we actually become weaker.
Lifting heavy, as an example, one day means that we can't lift the same amount the next or even the one after and we need to wait until the adaptations kick in before we can increase that weight (I am, here, grossly oversimplifying he timeline and processes to give you an idea).
In your case you faster (a systemic stressor) then started eating energy-rich food which is, again, a stressor while training and engaging in repetitive activity (which is your music playing). It's a perfect storm for all sorts of underlying issues to emerge. Be kind to yourself. Have a break from physical activity (to allow your body to fight underlying inflammation), start eating more healthily (reduced sugar and fat and, even if you can, meat) and get into exercise slowly but regularly.
I really hope this helps.
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