If it’s a minor issue, you can always call a doctor from one of the neighboring clinics. However, if it’s in the middle of the night or over the weekend when most of the clinics are closed, you can try calling 116 117. They will send a doctor to come and see you personally.
However, if it’s a life-threatening situation, do call 112. This is the emergency number in Germany, akin to our 995 in Singapore.
If you do go down to the doctor, remember to bring your proof of health issurance (Gesundheitskarte) and your passport.