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.
Here's an article with a video on push up form: the first part of his speech is a person's tendency to push their head forward as they get tired - doing this myself, I can feel the tension increase on my neck and cause the strain you're describing.
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.