Learning Russian is sort of like taking up a new sport. If you haven’t been keeping the muscles used for your new sport in good shape by playing some other similar sport, it’s very likely that you will get very sore the first few times you do it.
You have muscles in your mouth that you…
…simply don’t use when speaking English, and believe me, they will get sore when you try to use them to speak Russian.
Many of my friends who have learned Russian told me that their mouths got sore when they first started to speak Russian, and mine did too. It’s not a big deal, and it shouldn’t keep you from learning, but it may be a sensation you notice that lets you know you are doing something right.
If your mouth doesn’t get sore when you first start to speak Russian, it’s possible that you already have well-developed mouth muscles from learning to speak Polish or some other language.
If the only language you can speak is English and your mouth doesn’t get sore at first, then it’s very likely you are doing something wrong. You probably are using an English accent. Try concentrating harder on making your words sound exactly like the native Russian speakers you are copying.
The soreness isn’t a big deal, and it won’t last very long. But look for it. It can tell you if you are doing something right or wrong.