Hi there!
There may be many reasons why your shoulders appear uneven. There may be an imbalance between muscle strength in the muscles of your shoulder, around the scapula (shoulder blade) or muscles of your back. It may even be caused by a difference in the length of both your legs, or even by a tilting of your pelvis. From your physique, it looks as though you workout, so that can be a potential cause as well if not done right. You have mentioned that you feel tightness across your upper back and shoulders, but haven't mentioned if it causes you pain as well.
The best thing would be to personally visit a physiotherapist who can assess you and then see what is the real cause.
However, in the current scenario with the coronavirus pandemic, that may not be feasible for you, so it is best to get an appointment as soon as things start getting better.
What you can do at home right now though, is improve the mobility and movement in your upper back to relieve that tightness. I'm not sure if I can post links here, but you can easily look some up and start on a few that you feel comfortable with.
Also, their might be people who tell you that the muscles on your left side are too tight, and that this is the cause. However, evidence has suggested that tightness does not necessarily mean shortness. So although too much exercise may have contributed to it, it doesn't necessarily mean that's the sole cause. Again, it's best to get assessed by a physiotherapist whenever it becomes feasible.
Regards.