If she's been on it four weeks, she should just be coming off her period? Totally normal.... And it wouldn't be implantation bleeding, it would be residual blood from her period.

unless you have unprotected sex (no pill, no condom) there is no reason to assume you are pregnant. The pill gives us lots of weird side effects that may seem a lot like pregnancy symptoms to people who are terrified of getting pregnant (I've been there).

Don't worry, you are doing the right thing.