Normally I'd say it's probably just something she ate, but the "slimy" part is what concerns me....
If it's slimy, sticky and pure liquid it's probably bile. If your dog is throwing up bile, it means that there is no food in her stomach to throw up... and this is bad. She will become dehydrated and continue to throw up. You have to get her to the vet right away for an anti-nausea injection and possibly even fluids.
Once she is hydrated and stops throwing up, they can start to figure out what made her sick to begin with. Monitor her closely after you get home and make sure she is drinking. She may be lethargic, but that's normal.
Put her on a bland diet for a few days --- a combination of white rice and boiled chicken --- and no people food or treats! Let her stomach get back to normal. If in about three days she hasn't thrown up anymore, slowly start mixing her regular dog food into the rice/chicken combo a little each feeding time. Getting her back on her regular dog food will take about four-five days.