What's wrong with my dog?

Hi. my dog Daisy is 8 years old and a chihuahua terrier mix. I took her out to potty this morning and when we came in she started yelping like she was in pain. I picked her up to check her paws, but they were fine. For the past ten minutes shes been laying down, but 7 times she has stuck her butt in the air as if shes uncomfortable. Also, she didn't poop last night or this morning, she hasn't been eating much. Can anyone tell me what's wrong?