4months baby constipation. In babies?

My baby girl is 4 months and 10 days old and she is completely on breast feed. She is getting more constipation from past 20 days and she crys a lot and tries to push out but she I'll not pass stools and she passes every 3 days or 2 days once. Doctor prescribed surcal ano cream and if I put cream mildly inside and outside using finger she I'll pass little and again she feels constipated and kindly pls tell me what to do to make her regularly poop without any problem? and should we need to take any test to check out whether she has any problem or it is common in babies?? she had her vaccination on Tuesday. I m scared to use applicator as she is too small and get hurt inside so I inserted my finger once for 1/2 inch inside her anal do it cause any hard for her.