Could I be pregnant?

I have a four month old son. I got the depo shot while my husband was away so if the depo shot works correctly I shouldn't be pregnant right? My husband and I had sex one time before the shot and we used a condom and I had a period since than but only one since I've been on the shot. I'm nausea and dizzy. Just like I was with my son. I'm freaking out here. Funny thing is if I turn out to be pregnant that means we tried for 2 years to get pregnant before my son and it only took 2 months on birth control to get pregnant this time. How ironic would that be