Just wondering is there any problem with my breathing technique or due to health problem. Every time, after practicing for 20-30mins. I can’t carry on because I’m short of breath & especially after blowing those notes with longer values for e.g tied dotted minim. Then, I’ll need to rest as I feel like I’m going to faint & giddy perhaps it may due to my low blood pressure condition. Anyone has similar experiences just like me… feel free to comment or any advice.

Occaisionally I sometimes get a little giddy and my vision goes cloudy and black for a second or two, but that's it. Try just holding your breath for as long as possible; when it gets difficult and you start wanting to breathe out and take a breath, count to 5, then do so. The feeling of needing to take a breath and being out of breath fades after a while; it's not about the fact that you need oxygen, it's about carbon dioxide building up which makes you need to take a breath, so if you can slowly get yourself used to a little bit of it ... well, works for me!

You're a flautist, aren't you? I think it's likely that you are hyperventilating, or overbreathing. On the flute, some of the air just flows over the top of the instrument rather than down it, and this means you need to take a breath rather more often than I would on my recorder, where more air goes down, and much more often than an oboist would, where all of it goes down through a very narrow gap. That leads to you getting rid of too much carbon dioxide, causing dizziness, sometimes tingly fingers, and strangely also a feeling of being out of breath. I know that this is a common problem, but I believe it does get better with time. I'm not sure I have a cure for you in the meantime though! Try playing in short spells and perhaps in between practice fingerings without actually blowing down the instrument?
Any other ideas, anyone?