Its hard to open my jaw sometimes why?

[jasonmedly]

For some reason, this never happened before, but for a about a month now, when I eat or yawn, its not that easy to open my mouth wide. Once I open it I can open it again fully no problem, but sometimes it hard, now its not so hard where I cant do it, its just feels like something is there, and again this only happens a few times, like right now I can open it just fine, but just a few minutes ago, it was hard, and It felt uncomfortable a bit, just hurt a little. Like it gets tight if i keep my mouth closed and then when I go and open it its hard, and hurts a bit, but then I can open it all I want, until I close it again for a few seconds. WHY?