It could be a sub-clinical viral infection - causing muscle pain
It could be loss of electrolytes (low salt ) from loose stools.
See your GP - have Blood count / ESR / Electrolytes / and Stool analysis
Beware of imodium
- especially, if there is fever as also blood, pus in stool.
You will need a lot of fluids wen there is diarrhoea - dehydration
could contribute to your picture