Can u get pregnant if u have sex on ur periods and on the implomon?

Unlikely : If you have the Implanon already in and didn't just have it put in a few days ago then you are protected from getting pregnant, whether you are on your period or not. Don't worry.

5 in 10,000 chance: The failure rate for Implanon is 0.05% which means 5 in 10, 000 chance of not working. To give you an idea of how good this is -- compare it to tying your tubes [0.5% failure rate, or 5 in 1, 000 chance of failing], or iuds have a failure rate of 0.2-0.8% [2-8 in 1, 000 chance of failing].