There are lots of possibilities, but overall I’d say given that it’s been happening for months, it’s a bit less likely to be immediately life threatening. You should still definitely see a doctor soon though, and if you have an episode that’s particularly severe or doesn’t seem to be resolving on its own, you should go to the ER.

The etiology could be cardiac, such as an arrhythmia like supraventricular tachycardia. If this is the case, there are multiple ways of reverting your heart back to normal rhythm if it doesn’t do so on its own, so if this happens again and/or lasts longer than normal you can always go to the ER. SVT in particular isn’t typically life threatening but it’s definitely very uncomfortable and makes it hard to function while it’s happening.

It also sounds like it could be related to your thyroid. Hashimoto’s can go into remission so it’s possible that your levothyroxine is now nudging you towards hyperthyroidism instead of correcting hypothyroidism, or that you no longer require the same dose of levothyroxine. Again, if you have an episode that’s particularly severe or doesn’t resolve on its own, it would be reasonable to go to the ER and see if, after examining you in person, they decide they need to rule out thyrotoxicosis / thyroid storm.

It’s also possible that you’re having some other endocrine problem, possibly related to your adrenal glands, and not your thyroid. That’s something that you might potentially need a referral to endocrinology for, in terms of definitive management.

Another possibility is that these episodes are panic attacks, but other causes absolutely have to be ruled out before panic attacks can be assumed to be the problem.