OK, I have lots of experience with constipation in babies. My first piece of advice is get it sorted sooner rather than later, otherwise she will end up with-holding her poos because she knows it is going to hurt.
Glycerine soap around her anus as a first thing to try.
DD1 got horribly constipated and a glycerine suppository was amazing. When I told my mum about it she said her granny always recommended Pears soap, and I guess it;s the same principal. I have no idea whether ordinary soap would do the same trick, but I would give it a go. It relaxes the muscles, so the big hard poo can then get out - it's very instant, rather than waiting for the right food to do its trick.
Also clockwise massage on her lower stomach (must be clockwise). Also lie her on her back and do bicycle legs and also bend her knees and push her legs back onto her chest. While in this position you can rotate her hips. Does that make sense?
These are tips for relievng the immediate problem.
Next you need to massively increase her fluid intake. Orange juice is supposed to be very effective - or even brown sugar dissolved in water. Then of course lots of fruit, but especially prunes and prune juice.
You can buy infant glycerine suppositories over teh counter The problem is that if there is a big hard poo right at the entrance then it is really difficult to gte them in. If you can't then you could try snipping the suppository to at least get some of the glycerine around her bottom, or take her to the GP or a walk-in clinic to get them to put it in. It's fairly quick acting.
Good Luck!