My 13 Yr Old Shitzu/terrier Mix Has Been Vomiting Sometimes

My 13 yr old shitzu/terrier mix has been vomiting sometimes with diarrhea and sometimes w/ constipation off/on for 2mo. Today her b.m. started off runny/gelatinous the a small dry piece then back to runny. I've had her on a rice and ground beef diet. She's drinking well but very lethargic. Inbetween these bouts she seems fine and energetic. Currently unemployeed and can't afford alot of tests. Need advice