Painful dog.

I have a small dog, under 20 pounds, that I rescued off the street in Mexico. I'm not sure of her breed or age, I'm just guessing. I have had her at my home for several months, and before that she was in our rescue shelter in Baja. She is normally very healthy. Every once in a while she would suddenly scream/yelp like she was in terrible pain - she would yelp for maybe 10-20 seconds, then she would stop and be fine. It has not happened in a while and now she is suddenly yelping in pain nearly every time she moves. She does not wince or react when I push on her tummy, ribs, backbone, etc. She is eating and drinking and wagging her tail - not acting sick - just sometimes when she moves. Other times when she moves to curl up and scratch or something she does not yelp. I cannot figure out what is causing her pain. She has peed today, but not pooped. I did find some diarreaha on the florr this morning, but it could have been from my other small dog - not sure. I'm not sure if she pooped in the last two days as I was back in Baja working at the shelter and she was staying at a rental house with a babysitter, so I only saw her at night - and she was playing and running and just fine. She does seem to walk with a slight hunch, she is a long dog, and her back legs seems a bit stiff and awkward when she walks - it's very slight, but makes me wonder if she is constipated. She ate some fish and squash a couple hours ago for fiber. I've been giving her a small dose of prescription dog pain killer for the last 24 hours. 25 mg of Tramdol that is 1 year expired (I just had it on hand). I gave her one dose last night, one today and one tonight.
I appreciate your affordable, service, and look forward to your input.

Thank you so much, doctor. I will take Honey to a vet in Mx today - I live in San Diego and the vet care is much more affordable just over the border. I left you a small tip Thanks again for providing this service!