I used to be able to do about 50 push ups at a time with no problem when I worked out semi-regularly. I stopped when suddenly while doing push ups, as I'd be getting near the end and straining to get those last few, the muscles in my neck (just under my chin) start to feel like they're really pulling/strained. If I stop doing them, I don't really have any neck pain or problems afterward but it's kinda scary while it's happening. I'm just wondering why might this be happening?
It sounds like as you push yourself to complete the 50 push ups your body is getting out of proper form.
Instead of pushing yourself to complete 50, push yourself to complete 35 or 40 with great form. Just like weight lifting, we tend to get caught up in numbers (bench x amount, etc.) instead of focusing on proper form.