Can I se an insurance company over pre exsisiting conditons when I was just diagnosed?

I was insured with another insrance company (which I only used once) and was laid off from my job 1 year ago. A month later I went to work for another company, secured insurance and pulled a muscle in my knee. When I went to the emergancy clinic, the doctor noticed unsual swelling in my joints and sent me for bloodwork and diagnosed me with rhemitoid arthritis and turned me over to a rhemitologist. After being seen for several months (running up a bill of over $700.00, which I had made payments on) I was told the insurance company said I had a prexsisting condition. My lab bills are about 400.00 also. I have run out of medication and my doctor will not see me till bill is paid in full. My ankles and other joints are constantly swollen , and I am currently going through 500 over the counter ibuprohen for pain that dosent go way. I takes everything in my power to get up and go to work every day