Hello, I am Dr Ola and I would be happy to address your concerns about your little Chihuahua. I am sorry to hear he isn't feeling well. Given his age, it may be difficult to determine if his pain is isolated to this teeth and/or any underlying arthritis or soft tissue injury from playing with his sister.
I would be careful with the aspirin giving his small size. Over the counter human anti-inflammatory drugs can cause stomach upset and even stomach ulceration if not given careful. Make sure it is buffered aspirin. Also, make sure that dose is appropriate. For example, if he weighs 5 lbs, he should receive 1/3 of a 61 mg baby aspirin with food every 8-12 hours.
The most important aspect of him being comfortable until Monday (when you can see your regular vet) is REST. Keep him in quiet space and don't allow any further play time with his sister with no running, jumping, climbing stairs etc. Simply take him out to use restroom and back to rest. It is likely that he has over-exerted himself that is contributing to this and rest will prevent further injury and make him more comfortable.
As far as his teeth go, I would change him to a soft diet for next couple of days. This can be be either canned food or, if he eats dry food, simply moisten with water for a bit before he eats. Make sure his water temperature is kind of tepid to avoid any hot or cold sensitivity!
Once your veterinarian sees him on Monday, he/she can determine what veterinary anti-inflammatory drugs need to be prescribed for arthritis and/or dental cleaning and extraction of affected teeth. Until them, just let him lay low, even if he protests!
I hope this advice was helpful to you. Please let me know if you need clarification on anything or have other questions about this. Also, please do not forget to rate my answer which I hope you found to be "excellent." You may do this by clicking "rate our conversation" below (Your rating is the only means of compensation I receive for answering your question). Again, let me know if I can help you further:)