Freaking out!

I stopped using my NuvaRing last month. Had a period the first week of November, tracked my ovulation on this app, didn't "plan" sex. We just didn't use precautions when it DID happen, and I just had a positive pregnancy test! I've never been off birth control, so I didn't know what to expect. But I didn't expect it to happen this fast! I thought we would be pregnant maybe in 4 to 6 moths! I can't help but wonder if the pregnancy will stick though. It just seems to easy!