It's quite an insulting pair of phrases. You use it to someone who you really don't like if they are behaving in a way that causes you stress. If someone is messing you around, causing you trouble, you can say to them "you are a lot of trouble". For example, if someone is spreading rumours about you, they are "a lot of trouble". "You are a lot of work" means similar. It means that they make you stressed, cause you unnecessary worries, you have to go out of your way to fix things that they have done, etc. You wouldn't use this to someone unless you didn't like them and you want them to know exactly how annoying they are. Possibly, you meant to say "you are a PIECE of work" which is more like "you are a lot of trouble" but even more negative and harsh.This is used for someone who does bad things in a very sneaky way, is two-faced, and causes you deliberate but indirect harm.