Claims denial

Recently i was diagnosed with cancer and currently going through chemotherapy for testicular cancer. The treatment was pre-certified by the insurance. I started getting mails from the insurance company for each claim asking more information. I think they are trying to evaluate if this was a pre-existing condition.

I took a new job this year and the insurance started from July 1st. I was out of country from Jan-May and was not working and did not had insurance during this period. I visited a doctor and had an ultrasound done on May 31st while i was abroad. The doctor over there said that we need to further investigate and make sure it is not malignant. He said let's monitor and do another ultrasound after few weeks then we can decide whether to further investigate. He told me not to worry about it as testicular cancer is very rare and my case would turn out to be benign calcification or something. Out of thousands of patients he treat in a year he hardly sees one or two cases that turns out to be cancer.

I visited a urologist in July after i came back here. Surprisingly, He suggested to further investigate immediately and had a surgery in a week. A week after we got the pathology report and it turned out to be cancer and then started chemotherapy in october. The doctors notes do mention about my ultrasound done abroad in May. I visited a physician abroad before my insurance started but I was not diagnosed with cancer at that time. I was only told about the possibilities.

Now my insurance is asking me if i visited any physicians between Jan-May this year.

Can the insurance deny my claims even though pre-certified as i sought medical advice a month before my policy started? Can they treat my condition as pre-existing? How do they decide if my condition was pre-existing? If they deny my coverage can I fight against the company and make them pay for the claims?

I am really worried about the claims as the bills are huge. Any suggestions regarding this is really appreciated. Thank you.