Hello, I'm exactly the same, in fact tend to loose weight when very hypo, gaining a little fluid when slightly.
I must say personally I've experienced mild palpitations when stressed and not quite on enough thyroxine.
Being petite I find small changes make a big difference - 125 is too little for me 150 too much so have to do alternate days.
It might be worth asking for a check - you may even be on top much, though I find I get sweaty and only sleep about 4 hours when like this. And very loose bowels.
Actually bowels are my clearest indicator of correct dose. If constipated im definitely not right, too loose and it's too much. Forgive the question but how do you feel?!
It *might* be something else though such as b12, folate, vit d (might be low this time of year as mainly get from sun April- sept) or ferritin. So worth checking with gp.
I've been diagnosed with hypermobility syndrome which is possibly why'd don't put weight on, certainly my muscles are very quickly and easily affected by low thyrixine ( within range) and recently found low ferritin too, they feel weak very quickly.
I guess heart is muscle so could be easily affected.