Ouch! Help for next time!

For a little background, I'm doing a 10 week "Lean & Fit" program, so I stayed for BJJ class after my resistance band class...

So, we get to sparring, and I know I've got nothing left if the tank, and we worked just upper body in the resistance class, so my arms and chest are killing me.

I get set to spar a big guy, 250ish I guess? Short, real stocky guy, newish to our school, but I think he was with De La O for awhile.

He gets the takdown, I'm able to get guard though. He's holding my sleeve really tight, so I'm pretty stuck, his arm's down low on me, so I figure to try for a triangle at least.

I've got my legs up on him, not tight enough for the choke yet, and I think he pushes down to smash me... next thing I know, I'm loose and landing on my neck.

I guess he just plopped out? Anyone able to tell me what happened, and how to not let this happen next time? :)