Network monitor?

Hello all,

At my internet provider, there is a limit on the internet traffic per month. For some reason I exceeded the limit, although I have a secured network and did not download anything special (and was alone at home, no wife/children to do the things they do..).

I'm assuming somebody is downloading via our wifi, I changed the password and so on, but would like to know, if there is an easy program that runs on mac or on an ipad, which allows me to see what's happening on my wireless connection. This is from the perspective of the home user, so not to monitor the wireless traffic/who is connected/where... for an entire office building. Just home use.

Would someone be able to help with this? I'm not sure if this exists, but I would suppose it does?

Thank you