[ThatMan]

There could be many things but in my opinion whatever is, is expressed by the subconscious mind that way.

Before going into the spiritual and things like that, make sure your health is fine and that you do not suffer from some kind of problem, for example, your breathing may stop for a while at night and that is the experience you get because of it. If you wake up taking a huge breath of air this may be explained by a health issue, I am not a doctor, just trying to put things in perspective.

Then there's also trauma, a traumatic event that happened to you in which you could not breath, possibly something related with water. Notice that you are not drowning in an ocean, see, pool or something with a large quantity of water, but in your bathtub.

There could be something totally different, for example, things from your day to day life that may come up in your subconscious in the form of that experience.

As a last resort there could be supernatural things which are more difficult to quantify for there could be so so many things, sudden past life memory, having a nasty entity around that area, things like that.