My boxer is 16 but usualy gets around good. today she is falling and throwing up white foam. not eating?

She is panting as if hot and gagging on white foamy stuff. She just fell over in the yard this morning as if she couldnt hold herself up. I know she is old but she is always so active and strong. I dont want to have her put to sleep because of age, and I cant really pay for visit to vet today. Do you have any ideas what could be wrong with my baby?