Straining to produce a bowel movement can be seen with either constipation or with looser stools, unfortunately. But, it sounds as if her colon is irritated which is why she's straining and she's not really constipated.
While I understand that she's likely to be uncomfortable, I wouldn't necessarily consider this to be an ER situation..not yet, anyway. I have a few suggestions which may help today although if she's consistently painful when she defecates, then this probably requires further evaluation by your vet tomorrow, given her age.
1. Feed a bland diet of 1 part boiled, skinless, boneless chicken breast (or hamburg) to 4 parts 20-minute white rice. Feed only a small amount of one time, 3-4 times through the day. I would not feed any regular dog food. While this bland diet isn't going to be balanced for her, a few days will be fine.
2. If she is not currently taking any aspirin or nsaid products (although I don't why why she would be) and is not vomiting, then you can give Regular Strength Pepto Bismol. The dose would be 1 ml per 10 lbs of body weight 2-3 times a day. For example, if she weighs about 10 pounds, her dose would be 1 ml or about 1/4th of a teaspoon 2-3 times a day.
Pepto bismol should not be given to vomiting dogs since it contains salicylates(the active ingredient in Aspirin) which can irritate the stomach.
3. I'd start her on a good quality, canine probiotic such as Forti Flora or Resources Protegrity GI. These products can be extremely useful for gastrointestinal issues (and to help strengthen the immune system as well). They may be available at local pet/grain stores or can be purchased online or I'm certain your vet will have them, too.
Small dogs this age often slightly over-react to things but she shouldn't be painful when she tries to defecate as you know. But, at this point, it should be fine to wait until tomorrow to see your regular vet.
I hope this helps. Deb