Is this the left side or right, could be due to muscle strain, or even cold or flu, but if on left side, may be the early warning signs of heart attach, but these are usually followed by dizziness, blurred vision etc. So I would not worry too much, may be caused by strain from carrying something to heavy on one side. If pain continues, and accompanied with symptoms I mentioned above, best to get to an emergency. Otherwise, there are alternative medicines you can use
