Diarrhea in chihuahua.

Hello. I need some help in trying to diagnose my 4 month old chihuahua puppy. Let me give you a little history first and then I'll tell you what the vet has said/done.

For the last month or so she has been having episodes of diarrhea and then she will only poop mucus. She will run to the wee wee pad as if she needs to "go" but then she'll just leak out a clear fluid. Sometimes the mucus is a yellowish brown color. At times it has contained a small trace of blood. Along with the mucus, her tummy rumbles and makes all kinds of squeaky sounds (like IBS gas pains). She acts totally fine... Plays, runs around, etc. she is drinking fine.

I took her to the vet the 1st time. I was told the mucus was her intestinal fluid caused by diarrhea. I was told to give her chicken and rice. After a day. She was ok. About a week later symptoms returned. Took her to the vet, she tested positive for giardia. She went on meds for a week and seemed ok. After 2 weeks or so the symptoms returned. She was tested again for parasites but it came out negative. We discussed switching her food. She had been on royal canin chihuahua puppy mix. I bought blue buffalo puppy turkey and potato for sensitive stomachs. She's been on it 2 days and last night, she had the gas sounds and mucus leak again. I read online that she may have a food allergy and that switching the food could take a few weeks to see results. She hasn't really been itching but she does drag her tush on the floor. She has also had several deworming medicines, so I don't think that that is the issue. I feel so bad for her. I dunno what else to do. Any suggestions or help would be great. Thank you!

Ps she rarely EVER eats people food but the day before the last symptoms she ate 3 pieces of pasta that I accidentally dropped on the floor. I wondering if maybe that happened and she just has a really sensitive stomach. One previous time before symptoms, she was eating a rawhide. The next day she had the symptoms. This is leading me to believe it may just be a sensitive stomach and strict diet required.