They could be symptoms of the same problem, but there is no way I can make this diagnosis online. I would suggest that you first try and treat the diarrhea, and if this gets better and the weakness remains, then you will know that they are not linked. Also, if the diarrhea does not get better, then you will know that you need to take her to the vet.
To treat the diarrhea, start by starving the dog for 24 hours (if she is full grown, but not elderly). Make sure she has access to clean water all this while, so she stays hydrated, but make sure you give her no food. This will clean out her stomach and if there is a mild infection it will be wiped out. After 24 hours, start giving her a small meal of boiled chicken and rice which is easy to digest. If the diarrhea has completely stopped by this time, then you know that the weakness is not connected to the diarrhea.
To treat the diarrhea, start by starving the dog for 24 hours (if she is full grown, but not elderly). Make sure she has access to clean water all this while, so she stays hydrated, but make sure you give her no food. This will clean out her stomach and if there is a mild infection it will be wiped out. After 24 hours, start giving her a small meal of boiled chicken and rice which is easy to digest. If the diarrhea has completely stopped by this time, then you know that the weakness is not connected to the diarrhea.