I did some research for you on the Internet and found that there is little if any data available on the usage of Mersyndol Forte during breastfeeding. The paracetamol included in the medication does pass on into the breast milk - So the usage should be discussed with your doctor in order to evaluate potential risks.
There is a potential risk of overdosing your baby with morphine if you are what is called an ultra-rapid metaboliser of codeine.
Best way to approach this issue is to talk to your doctor to see whether it is okay to take that medication or whether the doctor has another way to treat your pain.