I've been using a Respironics Comfort Gel nasal mask. I also have a full face Mirage mask. The full face is weird to me. I like using the nasal mask but I know that I have a big history of mouth breathing even with deviated septum surgery a couple of months ago. I've been using a chin strap to keep my mouth closed during the night as I use my nasal mask because again I like that mask better than the full face. My question is is it possible that I stop breathing all together because my mouth is closed and I'm not breathing through my nose? Would I notice? Like wake up freaked out, unable to breathe that kind of thing? I don't wake up sweating like I used to when I didn't use my machine but I get the feeling that nothing is happening "therapy wise". I'm not sure if this is making sense.

Apnea sufferers do stop breathing through both their nose and mouth -- that's an apnea event occuring, you're not breathing.  The next attempts to breathe, if you have Obstructive Sleep Apnea, will be through either nose and/or mouth, it depends on the person.  It's all while you're asleep so you wouldn't know   Your CPAP will make sure you can get air through your nose if you need to, provided your pressure is correct and your mask is not leaking or you're not mouth breathing.

If you have a fully data capable machine you can check the display for your nightly AHI (the average number of apnea and hypopnea events the machine did not resolve) as well as your mask leak rate.  A high leak rate with a nasal-only mask system is usually due to mouth breathing.  (Not always, but that is the most common reason.)  When a high AHI is coupled with a high leak rate, it is usually because the leaking is sufficient to cause loss of therapy pressure and more apneas/hypopneas occur.  The machines can compensate for a fair bit of leakage but not 100%.

Some machines are compliance-only recording (e.g. how many hours you used it), so if you're not sure whether your machine tracks AHI and leak let us know the exact make and model, someone here can tell you whether you can find this information.

If you don't have a fully data capable machine then you may have to experiment with your full face mask or ways to keep your lips sealed.  (Search this forum using the search bar above, or the google-style search in the upper right.  Lots of ways have been posted, to deal with sealing lips.)  Or ask your doctor about this.  It is possible he/she could get you a loan of a fully data capable machine for a few weeks, to find out what is going on.  

Blessings,
--pseudonym

P.S.  If you just got the machine and have insurance, and are still in the 'rental' phase of the machine, it is quite possible you can trade up the machine for a fully data capable one, if that's not what you have.

Blessings,
--pseudonym

stiffkittens have you tried the Hybrid mask or others like it, I could never get used to the full mask, the bridge of my nose just couldn't handle it, no matter which one I used! and with just using a nasal mask, well I just couldn't keep my mouth shut, ( I have been told I have a problem with that in other areas of my life also but that is a whole different story!) but even with a chin strap I would still have air leaking out my mouth after I fell asleep, my lips would flutter in the breeze! Anyway the Hybrid has been the answer for me, I have modified it a bit using hair elastics, and some plastic clips, but it keeps the nasal pillows in place and I put just a dab of nasal saline gel in the opening of each nostril before putting on my mask and it works very well for me, I posted an updated picture of my mod, on the Hybrid mask forum topic. Anyway just another alternative to check out.