1) Terrible news when citizens start taking up arms. They won't go quietly into the night, and they'll be difficult to manage even if Russia takes over the country.
2) Their system of currency, the Ruble, is plummeting thus enraging many Russian citizens.
3) Thousands of brave people were on the streets in Moscow protesting what Putin is doing. These people can get up to 20 years in a terrible prison, for speaking out against their government. Could you imagine living in a place like this? I would like to think I'm as brave as these protestors but I'm not sure if I'd risk the 20 years in prison.
4) Huge protests are taking place around the world and Russia gets weaker with each one because some of these governments are trying to figure out how to make their constituents happy. Countries who normally wouldn't donate money to a cause outside of their own country, are sending funds and supplies to help the Ukrainian citizens. There's no way that a man like Putin thought that so many people would get behind Ukraine.
5) The internet is killing Putin. He's tried to shut down Russia's wide area network, so his citizens won't know what their country is doing, but it's just too hard to accomplish at this point. Working with the internet is a way of life for many including many businesses in Russia.
Anyhow, the above are just some of the reasons Putin is probably getting worried especially about what he'll be able to do in Ukraine when the citizens don't support him.