Actual "Flu" or "influenza virus" and the "stomach flu" (an enterovirus) are two totally different things.
What you are describing, with the vomiting and diarrhea would be classified as a "gastroenteritis" not as "flu". The "flu" virus causes a respiratory illness, cough, exhaustion, fever, and body aches (among other things) and may rarely cause some nausea...but not the vomiting and diarrhea. "Flu" tends to start it's run around this time of year, peaking often around January/Feb.
As for this "stomach bug"...they can be rotten. We had a major outbreak around the same time last year at our office in Maryland. Hopefully it won't last too long. Try to get some fluids in her..even if it's only sucking on an ice pop, or you can make your own ice pops with pedialyte. If she can drink and keep it down...JUST small sips at a time. Water actually tends to make it worse (ricochets right back up!), so stick with things like pedialyte, gatorade (diluted with 1/2 water so it doesn't make the diarrhea worse), ginger ale, etc. When she is able to eat again...go slowly with the BRATT diet (Banana, rice, applesauce, weak tea, toast.)
If she is vomiting and having diarrhea for more than 24 hours and is unable to keep anything down, if her mouth appears dry and she's lethargic...take her to the ER so they can get her rehydrated. These viruses can be just miserable. AND be sure to use disinfectant wipes on everything! They tend to be VERY contageous!
Hope she's feeling better soon!