How can I tell if Seroquel is causing my depression?!

My doctor prescribed Seroquel for insomnia and depression. He started at 50mg and slowly had me increase the dose over several weeks. A week ago, my mood suddenly dropped. I became very depressed. He told me to increase the dose to 200mg. It had been 150 before. I got better, but was unable to sleep. This continued for a week. A week later, the same thing happened, but it was more severe. I became very depressed again. I call him again and he tells me to increase the dose to 300. He said we would see the full effect of that change in 3 to 4 days. I took the 300mg last night, but now I feel equally depressed. I'm starting to suspect that the Seroquel might be causing these recent episodes of depression. They seem to have gotten worse as the dose has increased. The only definite positive change I have seen has been improved sleep, but that stopped after I increased the dose again.

During the time that I have been on Seroquel, I have had sudden depressive episodes like this once or twice before. This recent episode makes it 3 or 4 times in total.

I don't get to see my doctor very often. I'm not sure If I can endure the rest of the week before I can ask him if seroquel is causing this. Has anyone experienced this same pattern with seroquel?