I have an cancer, will a streptococcal b infection cause serious complication?

It can. This bacteria is not usually invasive for people that are not receiving immunosuppressive medications, are not pregnant or elderly or have poorly controlled diabetes. It colonizes the gut and the perineal area. With reduced immunity it can invade the blood stream and caused serious infection. The good news is that it is still very sensitive to plain penicillin.
Potentially. Any type of infection can be serious if your immune system is weakened from cancer chemotherapy drugs.