Definitely go to the ER if you get any bad pain or vomiting - if it ruptures it'll need to come out immediately. 2cm is not big, so don't worry too much about it rupturing as it most likely will not.
Did they say what sort of cyst? If it's simple (fluid filled), nothing will probably happen. If it's complex (fluid and solid, or just solid) then you'll most likely have to have another ultrasound 6-8 weeks after your first one. If it's still there then it may need to come out.
How are you holding up anyway? Are you feeling OK?