I am breastfeeding my 6wks old. Had unprotected intercourse with 5 month old sperm. Have not noticed cervical mucus. Its unlikely that I conceive, right?

Does not add up. What you mean by having, "had unprotected intercourse with 5 month old sperm"? If you have any concern, do home pregnancy test about three weeks after sex. Use first morning urine and follow the instructions for the test carefully.