Let her get away with it... once.
Don't do anything. Treat it as a one-off and give her the benefit of the doubt. Maybe she really did mean to pocket your tip, maybe she didn't count the money she was given and assumed people hadn't tipped, maybe people thought they had tipped but actually had calculated wrong. There's no way to know for sure at this point.
The point is, even if it was intentional, bringing it up with your manager doesn't help anything. It's not really your manager's business, and bringing it up just makes you look unprofessional.
Next time, take preventative measures. Work out your contribution out loud:
"Let's see, my meal was $20, plus two $5 drinks, that's $30. How much are we tipping? 15%? Okay, so that means I owe what, $34.50? Has anyone got 50 cents?"
You've explicitly mentioned the tip, so if anyone has forgotten they are reminded, and if anyone disagrees with tipping or with the percentage they can say so. I used to do this when I was younger and had a group of friends whose default was not to tip (whereas my default was to tip) and it works reasonably well.
When people are adding up the bill, you can remind them again if necessary. For example, if they're totalling to "$100" and that's the sans-tip price, you can gently correct that to "$115 with tip". If they continue, at least make sure that your tip gets passed on and doesn't go towards reducing the price of a coworker's meal:
"Well, $4.50 of that was my tip, so at the very least the cash should come to $104.50 even if no-one else tips!"
(This could come across as aggressive if delivered as so - make sure to say it in a friendly way and you'll still get your point across without sounding mean.)
If she somehow manages to get away with it a second time, then you can bring it up with her (I still wouldn't recommend going over her head and reporting her to the manager - it seems like overkill, and it might just be an honest mistake about differing expectations) and see what she has to say.