This is kinda long I know, but I would really appreciate some help here.
(Posted in Injury forum as well because I couldn't decide which)

So today I went to test my 1 rep max bench press to see if my strength has improved at all since beginning my workouts, with the aid of my brother as a spotter. I made sure to stretch off and get the blood flowing with some pressups, and did a few reps of about 120lbs to warm up.

My first attempt was at 165 lbs, which I thought I wouldn't have too much trouble with since I can bench 120lbs for 10 reps. And it went ok, it felt heavy but I managed it.

For my 2nd attempt I set the bar at 175lbs...and once again my bro helped me lift the bar off the rack until my arms were locked. So I took the weight of the bar and he let go, and it was only when I started to unlock my arms and lower the bar that suddenly my left arm collapsed along with a kinda muffled crumpling sound from my deltoid (not the sound of bone, but I'm thinking ligaments maybe?). It wasn't loud enough for my brother to hear.

He didn't catch the bar quick enough because he wasn't expecting any difficulty until the push, so it kinda bounced off my chest but that didn't hurt.

It doesn't hurt to have my arm at rest, it just feels normal. But certain movements that involve the front of my deltoid (such as holding my arm out straight) cause sharp pain. Not so bad that I can't do it, but pretty uncomfortable. Also there is no swelling.

Does anybody think they know what's happened, and how it's happened? And how I should deal with it? (Eg. rest it, keep stretching it and moving it around?)

Nothing like this has happened before, and it's not like the weight was way too heavy for me either. My left arm is weaker at the bench press as I have found out in the past, and with the 165lb lift my left arm did struggle to push the weight up.
I really don't know how to avoid this happening again, because I'm pretty sure my bench press form is correct.

Advice would be appreciated.

well before anyone can tell you what you should do it would be appropriate to at least pinpoint what has happened.

So it only hurts when you flex your arm (what about abducting etc), try supinating and pronating the arm in that position, does it hurt more?

Yes, it hurts when I flex my bicep, and I'm thinking that by abduction you mean raising my arm out in front of me keeping it straight? This also hurts as I said in my first post. To roll my arm so that palm faces up and then down only makes it increase in pain if I try to twist my arm as far as I can in each direction.

I hope this helps.

by abducting i meant out to the side...

If it hurts around the 15-90 degree angle this is the "painfull arc syndrome" and would probably have something to do with the supraspinatus muscle which is part of the rotator cuff.

But from what youve said i dont know, sounds like maybe you strained a muscle. Or maybe you have the inflammed bicipital groove where the biceps tendon is enclosed.

I think I recognised that crunching sound, I got it in my neck once and it turned out to be a strained muscle. The best remedy is rest for a week or two and then see how it feels