The best thing I ever learnt when it comes to brewing beer is 'don't worry' the second thing was 'if you are worried have another beer, repeat until your no longer worried'
Lots of people myself included primary ferment for at least two weeks! Before we even begin to look at our hydrometer. Sometimes your brew will bubble like crazy and other times it will go mad then look like it's stopped. From what you have said you had at least 7days of good positive yeast activity in your bin. I find this to be normal with most of my batches they go mental for up to 7days then calm right down and slowly plod over the finish line so don't worry about it at all, your doing the right thing checking it with your hydrometer and as most will tell you your looking for a consecutive reading over 3 days to know that your brew is ready. I don't think from what you said you need to pitch more yeast but I also don't think that doing so will hurt your beer at all.
To answer your questions:
I'm wondering what could have caused a sudden loss of pressure?
It sounds like your using a plastic bin for your fermentation. The pressure may have popped your lid and as the yeast calmed down the lid then sagged through gravity? again don't worry about it, I have had this happen and the beer tasted great and I'm still alive!
Also I'm worried about my beer becoming oxidized due to no pressure... is this possible?
Naaaa! Your beer had been producing c02 while it's been fermenting. This gas is heavier that air so it will sit on top of your brew like a protective layer stopping oxygen from contaminating your beer so again nothing to worry about!
In fact I've just been looking into open fermentation where they just leave the brew exposed to the air and let the c02 layer protect the batch! Interesting stuff.
So the short answer is your gonna be fine don't even worry about it, if it's your first brew I totally understand as I was like an expectant father when I did mine! But to be honest the worrying while un-nessersary made the brew taste all the better when it came out tasting awesome.
Good luck and don't forget have another beer!