Feel like I can't talk while stoned?

I smoke medical marijuana on a nightly basis. I've noticed that when I get high, I don't like to talk...at all. If someone asks me a question or says something to me, I feel like I physically can't say more than one or two words, if that. Its like theres something stopping me from speaking. I'm normally a quiet person when I'm not high, but when I'm high its like I almost lose the ability to speak completely. When I do talk while stoned, it sounds forced. I've known other people who have had this problem when they're stoned as well. How normal is this, and why does this happen?