I sent a part of my bitcoin to site using Electrum.

The site's receiving address was 1LGVk7yPaFhf9GJ8Kdfb9kqP6sNxy4DxVR which I posted as the send to address, but now it shows that $.38   went there and the rest (Over $500.00) went to 17CQ1dQXMmVTqfv3RKvZtsrHbXBgKFfCE6.

I never entered this 2nd address and the site I sent to says it's not their's. This was generated in Electrum so has someone stolen my money from there the same time I sent out my payment?

How can I find out who the 2nd address belongs to and get my funds back into Electrum. I should have not gone with Electrum as they provide no support.

Dragontail58