I am sorry to hear about this concern.
You must proceed very carefully. Th drooling would suggest possible nausea or a intra-oral (mouth) problem. If due to nausea, then the source of the pain and hunching over would suggest something inside the belly. However! For the breed, we cannot rule-out spinal problems.
Now, aspirin might slightly help musculoskeletal issues, but it could be very dangerous to give if there is stomach distress.
So, without question, it is time to get in to a vet for evaluation.
Clearly, none of these symptoms is typical, and even though it seems to resolve with aspirin, it shouldn't be occurring at all.
My primary worry is for the spine, and therefore, more appropriate anti-inflammatory therapy and pain medicine is warranty.
Well worth a vet trip.
Hope that information helps.