You are at least 25 % over your infamy level, presumably a lot more.
http://www.eu3wiki.com/$PROVINCENAME$_Revolts!
When you conquer land too fast, you accumulate infamy (also known as 'bad boy'). If you go a fair bit above the infamy limit you'll start to get some severely crippling events, among which rebels are one of them. You are also at a risk of getting events lowering your stability and increasing your inflation. To paraphrase the tool tip: "Don't go over your infamy limit or bad things will happen."
Your infamy will decrease over time (unless you hold Unlawful Territory in the Empire) but you can also decrease your infamy by releasing some of your provinces as vassals, since this will lower infamy by -2/province, at least if the land is in Europe.