Constipation or tight butt? ??

Mimi
Hello eveyone. My baby girl will be 3 weeks tomorrow. When she was born, her bowel movements were very regular (2-3 yes a day). But for the past week and some days, she's been having a hard time going. I hear pushing to she is red in the face but nothing. We took her to the doc, who told me to stay away from diary, she also inserted her pinky finger up her butt because she thought it was tight there. It only helped a little bit. But she seems to be having a hard time going. I have been also inserting a q-tip coated with vasaline whenever she gets frustrated and that also seems to work sometimes, although not all the time. I would like to know if anyone has had this issue. She is breast fed and her poop is liquid when it comes out so I don't think it's constipation. But I don't know what it could be. Any tips on how to help her would be much appreciated. Thanks in advance!