Preface by saying I'm not a physiotherapist or anything like that, just an ameteur fitness and yoga enthusiast.

My first piece of advice after reading these comments is don't take physio advice from r/beatsaber. Seriously some really bad advice in this comment section. Find a more qualified expert, even if it's just a different sub which focuses on health/fitness/ask a physio type thing.

Secondly (and I say this fully recognizing the contradiction of telling you not to take advice here, so take with a grain of salt), soreness and tightness are 2 very different things. Soreness is completely fine and to be expected any time you increase your exercise.

Tightness is bad, and it can lead to injury. It sounds like you may be dealing with some tightness. I find that using a foam roller on my upper back helps when I experience similar tightness. Stretching or yoga can also help loosen those muscles, as long as it's done in a restorative way. Rest would also likely help but may not be necessary depending on the whether other strategies are helping.