Sudden pain in dog.

I have a nine year old female black lab mix, Polly. When I adopted her, I was told the mix was husky, but I've never been totally sure that was the case. She was one of two puppies in the litter that were born with an unformed or ruptured phthisical globe. The eye was cleaned out and the socket sewn shut. In addition to the missing eye, she does have a mass on her chest that's been there for a few years. The vet agrees that it's likely a lipoma, and it has never seemed to irritate her.
One morning about three weeks ago, Polly yelped as though she had been stepped on. She had been lying at the foot of the bed, then jumped up and looked at me as if to ask what had happened. She paced the room a bit, then circled and laid down. I went about my business, then it happened again, and then again once more. After the yelp, she would wag her tail and seemed to be in no pain. I took her on her regular walk, and while she was less energetic than usual, she was not lethargic or limping. Over the next few days, she was less energetic than usual, but not lethargic. She ate and used the bathroom as usual. I did notice a yeast buildup in her ear, which I cleaned and treated with drops. I also scheduled a vet appointment. She returned to normal levels of activity over the following week and a half or so, and showed no irritation around the ear. The vet did a regular check-up, did blood work for parasites, and she got a clean bill of health.
About four days ago, I returned home late it the evening. She jumped up with her typical energy, but yelped as she did so. Her tail was tucked but wagging, and she was shaking. I felt her over, but no specific spot seemed to giver her any pain. She repeated the yelp and tail-tucking twice more that night. She usually sleeps at the foot of the bed or on the couch, then wakes me in the morning. That night she slept on top of me, and was trembling almost constantly. In the morning and over the next day and a half, she continued to shake intermittently and displayed subdued behavior, but no yelps. Last night, she paced the bedroom, shaking her head. This morning, as we walked into the house after our walk, she yelped again. She continues to eat and go to the bathroom regularly. She does seem slightly dazed, and will sit and stare at me in a way that seems odd. She continues to have bouts of trembling, as if she's afraid. Do you have any idea what is going on? What could I ask a vet to look for? I'm very concerned, of course, and want her to be comfortable. Thanks for any insight.

Thank you for your quick reply! I've looked around her anus, but I'm not seeingany irritation. Polly is very nervous at the vet - she shakes from beginning to end. We've really only been to the vet to update shots, so they're not overly familiar with her. Will they be able to tell the difference between her shaking from fear and shaking from pain? If there is back pain, will it likely be something she'll have from now on?