Ahhh I totally get this, happened to me during my second pregnancy too, around that stage! It's usually normal because baby is pushing up on your lungs, tapi please mention it to your doctor next visit just to be safe, especially the "tertahan" feeling you described. A few of us mum friends found that sitting up straighter, doing slow deep breaths, and sleeping propped up with extra pillows helped quite a bit, also drinking more water sounds random but it does help with that breathless feeling sometimes. Fingers crossed it improves for you soon!