Hi--
I'm on isoniazid now, I couldn't answer before because I wasn't on it. After only a few months, I feel much better on it and know when I forget to take it because I start to feel bad.
The breathing problem sounds like an apnea or something and I had that same problem alot when my thyroid was out of whack. It was like I had to remember to breath and wasn't getting enough oxygen. I was diagnosed previously though withhypoxia, asthma and COPD, but the real cause was my endocrine system (thyroid pituitary and adrenals) I've found out with treatment.
I would definitely try to got to a doctor, even if you have to go to ER and get checked, because you sound like you are not getting enough oxygen or that maybe you could be having a reaction to the isoniazid, unless the isoniazid fixes it.
I can't believe how quickly I feel bad when I take a dose late. It is one medicine I don't forget to take.
Also, I am now having to get my endocrine retested since my temperature has risen and I have so much more energy now that I'm taking it.
Take care and let me know what you find out.